Obviously the more characters you play the less you are playing one character specifically. That being said, playing more than one character definitely isn't a bad thing. Playing other characters could help you learn the match up for when you play against that character with your main. Playing more than one character can expand your diversity of technical abilities (although brawl isnt that techy but im not going to get into that). Moreover, playing more than one character might even find you a new main or ,maybe an alt/2ndary char, or a character you might learn to use as a counter pick to a match up that may be particularly hard for your main, fill the gap if you will. You're only as strong as your weakest link. Let me know if you've got anymore questions.
Just for some background knowledge, essentially establishing my credibility, I'm mainly a melee player and I'm almost certainly top 15-20 (maybe even top 10 but its pushing it) players in western New York and I'm 17 years old, a senior in High School. I'm not tippy top of my play yet but I'm just starting to practice claw which may help in the future, I'm positive it'll pay off. I also find it fitting to mention that I live 15 minutes away from the 2nd/3rd best player in western new york and Ive been hanging out with him regularly, and I'm not really close to his level yet, but playing him will get me there. Just playing against him has improved my play significantly. My second favorite smash game is Project M for obvious reasons (I'm a melee player). Then Super Smash Bros. 64, offense ftw. Then probably Melee SD Remix, Super Smash Flash 2, Super Smash Bros Crusade, Super Smash Land, Super Smash Flash, Super Smash Clash (or Super Smash Bros. Clash), Super Smash Bros. NES, then Brawl+, Brawl-, Balanced Brawl, and finally...barwl. Obviously I am being facetious, otherwise I wouldn't have named off 13 unique Smash/Fan made Smash games just to make Super Smash Bros. Brawl my 14th favorite smash game. This being said though I am not a poor Brawl player for I am smart a smash player in general with good spacing and tech skill and knowledge, etc. Lets not forget to mention...I main metaknight...easymode...I'm just going to stop my rant here.
TL;DR
I am REALLY REALLY GOOD AT SMASH GAMES.
(or at least very knowledgeable about them)