Alright, the next person who brings May into this thread is gonna get heavy neg-repping from me. I'll add a scolding by PM if you're adventurous, too.
Just because Grimer appears in people's avatars doesn't give him enough recognizability. Allow me to muse on recognizability:
This press release from NASA explains how the character of Deoxys was used to teach children about viruses, DNA, outer space, the ozone layer, and extraterrestrials. Was Grimer used as an educational tool?
Lucario has been made into many more toys than Grimer has. Show me a Grimer toy and I might just change my opinion about him.
You say Grimer should appear because he's appeared in almost everyone's avatar. To me, that's bull. Show me a MASSIVE Grimer fanbase outside of SmashBoards and I might change my opinion of him.
Until then, fluffy, the only chance Grimer can get into Brawl is as a Pokeball Pokemon. At least his appearance will satisfy you guys.