Yes, fan art is definitely legal! There's a lot of misconception out there about copyright and intellectual property, but the fact is that fan art is protected under the law. Copyright law actually encourages creativity and allows for a certain amount of freedom when it comes to derivative works. So as long as you're not copying someone else's work wholesale or selling your fan art without permission, you should be in the clear.
Now, with all that said, it's important to remember that just because something is legal doesn't mean it's necessarily ethical. If you're making fan art of someone else's work, it's always best to get their permission first. And if you're selling your fan art, make sure to give the original creator a cut!