I think you should only draw fanart if you truly enjoy whatever fandom it is you have chosen to draw fanart before. A few years ago I was 110% knee deep into my own characters and they were the only thing I ever wanted to draw. I received absolutely zero feedback, which admittedly was disheartening at times, but I loved my characters so much I didn't want to give them up. Recently my childhood love for some of my absolutely favorite series, MLP and TMNT, have been sparked back to life, and suddenly they're ALL I want to draw! I adored both series as a kid and rediscovering them now as I'm older has lit a spark in me that has inspired me all over again. And you know what? I'm STILL improving even though I'm drawing fanart. I've started working a lot more with color and shading. I'm experimenting with new perspectives to make my pieces more dynamic for action shots. I've studied anatomy a ton more and my grasp on the male form has improved dramatically. In a way, working on fanart has jump started my progress as I'm a somewhat competitive person, and being able to see other people draw the exact same subject matter as I am has really been a kick in the pants to practice more than ever to try to reach their level. Sure, I have a LONG way to go and looking through my gallery maybe my progress really isn't that impressive, but I don't post everything online that I create, and besides, it doesn't matter if other people think I'm improving too slowly, I'M the one who knows when I am learning, I'M the one who knows when I'm pushing my boundaries and trying new things, and that's what matters.
Also, I wouldn't be so quick to judge that just because someone draws fanart for a very popular series, it doesn't mean they're always just doing it for the attention. Some of us genuinely love what it is we're drawing and it makes no difference whether that series is popular or not.
Like I said elsewhere, doing fan art is like being friends with a popular guy. If you genuinely like the guy, it will be fun. If you're only hanging out with him in the hopes that his friends will start liking you too, you are eventually going to end up resentful and disappointed, because it's really hard to ignore the fact that people are there to talk to him, and not to you.
Anyway, if you actually want to be popular and get attention, you're going to have to put in as much work as the popular artists. Nobody gets a prize for sitting on their ass and dreaming about winning first place.
On one hand, it is a good thing to do- it forces you to draw to a form already in existence, and people can accurately judge how close you are.
But if you don't want to, then don't do it. Art you don't care about doesn't feel the same. I only draw characters that I feel a genuine affection for, and people tend to comment to that effect. You can tell that I am just absolutely in love with them.
Honestly, I draw some pony fanart (along with some other stuff) for fun, and they have lately gotten 200-300 favorites or so, but the people who watch me for pony fanart don't usually comment on my other stuff.