Can a laser make for an effective space weapon to begin with?
First I will address some of the frame challenges that say a laser makes for a useless space weapon to begin with, since that seems to be a major point of contention. Depending on what technology you put in your setting, this could either be true or false; so, first, let's design a setting where lasers are a threat worth trying to counter.
While lasers do spread out some in space, they will still likely be the best option for long ranged attacks in most near-future settings. No, they will not hit something from several light seconds away, but that is not necessary. It just needs to out perform your kinetic weapon options to be your best option. As one other answer pointed out, a laser that fires at something 360,000km away loses 90% of its energy due to optical limitation... but that also means that anything within about 30,000km is still going to hit with most of it's energy. While some settings assume that space battles will take place in deep space at relativistic speeds and ranges of several light seconds, other more conservative settings posit that battles will only take place in the areas of space directly surrounding targets of importance where ships are required to match speeds enough to become part of the same general reference frame so that they can both make an attempt to "hold the ground" where the planet, space station, etc. that they are fighting over is. When you envision space battles being most fought primarily between planet orbiting fleets, two ships in a low orbit around an Earth size planet can accurately and nearly instantly hit any target it can achieve line-of-sight on meaning that lasers have plenty of range to be a sensible weapon choice.
In real life, unguided munitions (cannons, railguns, etc.) become pretty inaccurate at ranges of more than a few km; so, even with computer assisted targeting, minor material imperfections can cause a shell to drift several meters off target after just a few km. So, even if you can fire a shell that can deliver a more powerful, more concentrated strike than a laser, odds are it will not be able to hit anything small enough to matter at ranges of greater than a few hundred km. So, a laser ship could cook a gunship from way beyond its effective combat range.
Missiles can theoretically do better, but depending on the tech available in your setting, this may or may not be the case. If ships in your setting can not both accurately and instantly move at relativistic speeds, then this puts a cap on how fast you can make your missiles while still giving them the "reflexes" required to hone in on a target. So while your engine tech may be able to get up to relativistic speeds, it could lack the acceleration to get missiles up to speed fast enough to overwhelm point-defenses. Low Altitude Hypersonic missiles for example are hard to shoot down on Earth only because you have so little time to react from the point when a missile crests over the horizon to the time it makes impact, but in space, the same missile fired from >1,000km away will have no horizon to hide behind on its approach as it builds up speed. This will give computer systems and point defenses plenty of time to identify and respond to the threat.
As for suggestions about cheap reflective/refractive armor. There is already a known solution for this. Yes, there are material's out there that can reflect over 99% of light in the visible light spectrum, but there is no material that is super reflective at every possible frequency in the EM spectrum. If you make mil-spec lasers able to adapt thier light frequencies based on the enemy's paint job, you can adjust your attack to be absorbed by your enemy's specific defenses. Also, most super reflective surfaces burn at relatively low temperatures turning them black; so, even a low efficiency on the front-end could potentially turn into a higher efficiency as it starts to burn the target. Adaptive frequencies also allow the laser to find gaps where things that should stop the laser become transparent allowing the beam to go right through smoke screens that block visible light.
As for the damage potential of a laser, you do not need to actually do a lot of damage to incapacitate a space ship. Giant sci-fi lasers that cut ships in half causing glorious explosions is totally unnecessary in space. All you need to do is make a hole big enough to vent the atmosphere or cut a small hole in a vital system. A modern 25 kilowatt laser is adequate for burning a hand sized hole into a light armored combat vehicle AFTER passing through a significant amount of atmosphere. Since weight is a major concern in space, it is likely that armoring against a laser significantly more powerful than this would be grossly impractical; so, laser systems no more powerful than already exist that are small enough to fit on a truck will be all the firepower a warship needs. Assuming any sort of future tech, there is no reason you should not be able to manage even much more powerful lasers than this.
So now that we've established a workable laser META, will your lens idea counter it?
Unfortunately, there is no setting in which this would make since. A lens can not bend light without also heating up and absorbing some of the energy of the light passing through it. In school, many of us learned that light passes through a medium, refracting due to photons following some weird orbital or bouncing pattern through the medium, but this has been experimentally and mathematically disproven. Refraction requires light waves to stimulate vibrations in a medium. These vibrations create a wave pattern of constructive and destructive interference that form the apparent change in the speed and direction of light. So, if your medium absorbs no energy, it creates no vibrations and there is no refractive wave pattern... and vibrating atoms are the very definition of heat.
That said, a HEL's focusing lens can take a large lens surface perhaps a meter or more across and focus it down to just a few square mm. It is the focusing of energy that allows a laser to project an amount of energy that is low enough not to destroy the focusing lens, but then hit high enough to melt steel. When you focus this energy into another lens of similar quality near the target, it will rapidly heat just the tiny little point that it touches and create a thermal shock that will shatter your attempted refracting lens.
There is also the issue of positioning the lens. Lasers fire at the speed of light meaning that you can not know where it will hit until it hits you. This means that your lens would have to be big enough to protect your whole ship, but tough enough that every square millimeter of it can withstand the thermal shock of an HEL laser.
So what is the best defense against a space laser?
If you want to make a good defense against lasers, you want to use Starlite, or something like it. It is a light weight material that can coat your ship's armor and protect against lasers well in excess of strong enough to melt steel. Starlite based armor will significantly increase the needed power it takes for a laser to put a hole in a ship, but even if this forces you to put your lasers into the megawatt or gigawatt range, the addition of any future tech that would allow interplanetary warships to exist in the first place would inherently make the heat and power concerns of such lasers on a spaceship achievable... so you just need to assume that such armor is thick enough to matter, but the lasers are strong enough to still be useful: whatever that balance may be. Otherwise there will be no laser weapons, and you are back to ships using missiles and cannons anyway.