When you're in art school, you're plagued by teacher's opinions. At least I was. Not necessarily in a bad way, sometimes you need to be told that what you're doing sucks. But art teachers don't always give the best career advice. The quality of their advice, in my experience, depends on how seriously they take themselves.
Still, it took me awhile to disregard the impression I received from some teachers that illustration was not a real thing. It was something that was fine for the peasants, but if you wanted to be taken seriously, big galleries were the only real option. And so, for a few years I buckled down and tried to make serious art.
Last summer however, I had a day where I sat down at my drafting table, and all I wanted was to paint something silly. So I drew a chicken that was saying something offensive, and it made me laugh. Ten minutes later, the gallery owner (my studio was behind a gallery at the time), saw it on my table and informed me that it was fine if I wanted to do that sort of thing, but that I shouldn't expect anyone to hang it. I was a little crestfallen, but I thought he had a point. So I put the chicken away. Until the next day, when I got the urge to draw another chicken.
Fast forward a year, and my (different) studio is filled with silly animals and children's books. And I couldn't be happier with my art. It isn't serious, or perfect, or anything like that. But it's mine, I really enjoy it, and every day I can't wait to make more of it.
Moral of the story is: learn from school and criticism, but at the end of the day if what you enjoying making isn't as serious as people would like, that's cool. Make it anyways.