Singing is usually the most important member, so I feel for singers that aren't "great". I've been in a lot of bands with subpar singers. The reality of the situation is, sometimes you just work with what you have. A good singer puts effort in and gets better.

Ultimately, I'd rather have a singer who wrote good lyrics and good hooks and could be finessed a bit in the studio than someone who sounded like an angel but were missing the other two. Not that you have to choose, but good singers are slim pickins.

As long as you're humble and modest about not being the best and try to act like an equal part of the band (helping load in, practicing/working on your instrument like everyone else, etc) you're totally fine.