If you can see through it, so can the laser
@Daron's answer of "if it's dense enough and the laser's weak enough" is pretty accurate. I thought I'd need to do some math on the absorption of the dust particles and radiative area (the reason why dusty plasma is a good idea is because of their large radiative surface area to mass ratio, making them great heat exchangers), but the bold argument at the top is, in my opinion, pretty reasonable. If the microparticle plasma is only partially opaque, then that fraction of light that makes it through (e.g., sunlight reflecting from the vessel's hull, through the dust, to your eyes) is the same fraction of laser light that'll make it through (from just outside the magnetic confinement to the vessel's hull).
Actually, it'll be "slightly" worse than that. The dusty plasma will have an effective scattering length (what @Daron's answer describes), but as the laser bombards and the dust's temperature increases, the scattering length will increase until the plasma isn't scattering significantly at all (and the scattering length is already likely to be on the order of the dusty plasma's depth, anyway).
If you're being fired upon, it means you're probably within the effective range of a diffraction-limited laser, where spot size is likely on the order of a meter or less (a few cm spot size at 100,000 km distance is not unreasonable). Dusty plasma simply isn't going to do much against a 100 MW/m^2 pulse, as it's intended to absorb and radiate much, much less than that. Even if your dusty plasma shield was made super-dense and completely opaque, the principles of atmospheric hole burning can be used to quickly and effectively drill through it. Fire a powerful sub-millisecond pulse to ionize the first tens of meters of plasma shield, wait a few fractions of a second for the plasma to expand to near-vacuum, and then rinse and repeat until you've breached the shield and are now penetrating the hull. The first few milliseconds of beam power might have been wasted making it through the plasma shield, but the majority would have made contact.
(It's good to keep in mind that most of the laser's energy will be delivered in less than a second. There's not going to be enough time for the dust to "fill in" the hole before any competently powerful laser burns through and strikes the hull.)
You likely wouldn't want to surround your ship in an opaque cloud of magnetically-confined dust, anyway. Firstly, the upshot of dusty plasma radiators is their high radiative power to mass ratio, making them lightweight radiator options. Secondly, a cloud of the stuff dense enough to be opaque is going comparable in mass to a solid sheet of the same material laminating the hull.
Your best defense against lasers is likely to be some combination of ablative/NERA/composite/capacitive, etc. armor, spaced and angled. Especially angled, weathering the incident beam energy over a greater surface area.
TL;DR: dusty plasma radiators are good at being low-mass radiators, but you might want to look elsewhere for passive laser defense.