It's bad for an illustrator to be a perfectionist. It's bad for anyone to be a perfectionist. Perfectionism kills motivation and the momentum of imagination. It's like tying a big rock to the leg of a beautiful bird. You can soar - but, wait, what's that you forgot to do? You really should do that before you continue. It won't look right if you don't. People will point at it and see the glaring mistake you made because you forgot to do that thing. You need to plan ahead and foresee the problems you might face. Let's take a minute and do that. Then a few more just to be really sure. On second thought, if you wait til tomorrow maybe you'll be in a better frame of mind to draw this picture. (This is when procrastination kicks in. It tags along with perfectionism a lot. I think they carpool.) You need reference pictures and you can't find the right pose. Let's just wait and do that later.
And you know what? You never do, and then you proceed to kick yourself for it for days. This is why perfectionism and its cohort procrastination are bad for creative people. There's always something you could do better, wait for, improve upon. And if you wait to do it 'right', you might never do it at all.
We learn by doing, and by making mistakes. Sometimes those mistakes become 'happy accidents' and turn out to be the best part of a piece. Sometimes they take a story in a different direction. Embrace them. They are not out to get you.
Perfectionism is a boogeyman. It paralyzes you with fear, and when you're paralyzed you don't try new things, you don't experiment, you don't innovate. You don't step out of your comfort zone, because your comfort zone is what you know you're already good at. Heaven knows what would happen if you tried something you've never done before!
This brings me to the concept of newbism, the state of being a newbie. (Not to be confused with n00b, which is derogatory. N00bs purposefully annoy people and mess things up.) It's okay to be a newb.
I was insulted with that term recently while playing a new game online. I stared at the chat screen for a minute, made a face, and thought, "Well, duh! How else am I supposed to become a veteran?" Elite gamers, artists, writers, musicians, and anyone who's ever tried anything, are not instant. Everyone is a newbie at the beginning.
You won't become a veteran by being a perfectionist, unless your perfectionism allows you to better yourself without criticizing yourself. Self-criticism is a beast and tells you you just aren't good enough. Ever. Coupled with perfectionism, you might as well be wearing a straitjacket.
So if you're an artist, be an artist. Play with media, concepts, other styles, and don't be afraid to make work that isn't perfect. The best art isn't perfect. It's expressive, it's alive, and should reflect the artist who made it. Don't be perfect. Be yourself, because no one else can be you. Your work will follow.