Not that there's anything wrong with it. It can be fun in small doses. It's only really problematic when it's all a reader ever reads.
I know a couple of adult guys who've only ever read superhero comics (they're not big on actual books, either, outside of stuff like technical manuals—they're the type who dismiss all literature out of hand as pretentious twaddle) and they've got a pretty warped, hardline sense of concepts like justice, security, and criminal responsibility (naturally, they're big fans of vigilantes like Batman and the Punisher). And they're not stereotypical geeks, either. One even has a family and a stable, high-paying job and everything and is even slightly left of center when it comes to his politics. Start talking to them about criminal justice and rehabilitation though and they have all the intelligence and maturity of a ten-year old.
I'm sure their issues go beyond "they grew up reading nothing but superhero comics," but it can't have helped their maturity.