As some of the other responders have mentioned, you don't need to make your world less utopian if it suits your story. Consider the degree of darkness necessary for the story you are trying to tell. If adding darker elements like drug abuse, human trafficking, domestic abuse, bigotry, physical and mental disabilities, or war crimes doesn't resonate with the story you are trying to tell, why add it? I've seen some praise gritty stories for delving into the "hard topics", but I've also seen numerous others give examples of how doing so can actively subvert your plot and message and how it often feels like authors only add that to look edgy and topical. E.g., if your message is "life is better than you think it is, even when hope seems lost", having your setting being an utter hellscape full of cruel people actively destroys the message you are trying to send.
However, there is one incredibly easy way to make your worldbuilding less utopian: think about how people would screw it up. Literally every positive social and scientific advancement humans have ever made has resulted in nightmarish consequences because people will abuse that for their own gain.
- The same Haber–Bosch process, which averted a long-feared global famine due to nitrogen shortages, ended up being used to mass produce explosives that killed or crippled millions during the two world wars.
- Orville Wright, who helped invent the airplane to free mankind from the ground, lived long enough to see his invention used to bomb cities into rubble (he died in 1948).
- Gene editing, which has the potential to create drought-resistant crops and cure diseases, is already being abused to do things like genetically engineer plagues for warfare and population control, create transgenic chimeras, and create designer babies (i.e., eugenics). Most of these are still in the experimental stages, but a big concern among bioethicists is that most of these experiments are being done in places like China and no one has the leverage to stop unethical experiments in China.
People, in general, seem to be almost hard-wired to screw over and exploit others for their own gain, especially those outside their immediate social group. And indeed, we see this behavior a lot in pretty much every species of social animal, even ants and mole rats. Although humans can be kind, they are also lazy, and very, very selfish.
An easy way to pinpoint where these abuses would take place is to think about your world, think of the awful consequences your fantastical elements (magic, technology) would have, and find the spot where you go "no one would be dumb/selfish enough to do that..." Stop right there. Because eventually, someone would. Even if 99.9% of the population are ethical enough to not do something, there will always be some segment that will. Even if that act is as unethical as causing nuclear war.
Even utopian technology could be abused mercilessly. Take, for example, Star Trek. Replicators could be easily used to craft assassination weapons on-board a spacecraft, transporters could be used as torture devices (or worse, bomb delivery systems), Warp-capable spaceships make for good kamikaze weapons that can sterilize a planet, etc.
Another way to figure out where the problems will arise is look at the distribution of power. If there is any form of concentrated power in the society*, there will end up being corruption and abuse. I ran into this with a question of my own, where I asked how to keep an organization that upholds the supernatural masquerade heroic. I got answers which painted a pretty horrifying answer that any organization involved in policing the masquerade would rather quickly become corrupt and begin abusing its access to supernatural resources to oppress and control mortal affairs, like a CIA on steroids with no oversight from the government. People will always be awful. Life will always be unfair. Suffering will always exist. And as long as human beings continue to exist in the universe, humans will continue to make other humans' lives miserable.
"*" - And there will be. Even in an idealized society where there are objectively no distinctions based on class/race/gender/whatever, individual differences between human beings will still result in inequity and strife. E.g., if all differences due to wealth/education/other biases are eliminated, things like "pretty privilege" will result in some individuals systematically having more opportunities in life than others (and overall being more successful). And does anyone really believe that utopian technology would remove the petty drama humans get up to on a daily basis, like that seen in many high schools or offices? Indeed, if anything the more utopian things are the worse humans seem to behave, since there is no longer any reason to put one's differences aside and cooperate for survival.