well is it really all that bad if you don't have a main?
having a nice balance gives you more options to counter your opponents character (assuming you know who they pick)
and if you play the person several matches, you can switch up so they cant use whatever they learned about your fighting style from the last match
and its just alot more fun to play about 20 or more characters effectively because most people tend to just stick to 1
sure its nice to be the best at a particular character, but then the only way to go is down...
keep in mind its a party game so have some fun and stop worrying about things like this
^_^
also you might just develope a main on your own