It is impossible to change it. The ESRB auto rates m to any online game due to the players. It is as easy as posting a link in the chat. Then we get into TF2's skins and such.
Any game containing excessive blood/gore is rated M.
Custom skins are custom content. Custom content is not rated by the ESRB.
Communities are not a reason for it to be rated Mature either.
And "The ESRB auto rates m to any online game due to the players." doesn't really make very much sense. How about World Of Warcraft, hmm?