Something came up a few days ago in this thread, whereby someone stated that by playing multiple characters they were gaining multiple styles which would help them.
I disagree with both that person and KOH for supporting them. The only thing you gain from playing another character is their playstyle for when you play as them. Learning to play Bowser doesn't improve my ability with Sonic. It might help me learn Bowsers moves more and defeat him easier (but the same could be achieved by versing other Bowsers online, etc.) but it certainly doesn't help to play one character in the hopes you'll be better with another.
You may argue that experience with some characters such as Jigglypuff's float in, damage the opponent and float out style will help when you begin to learn Wario who does much of the same, and I agree. This doesn't apply to the likes of Bowser and Sonic though, who don't share similar qualities. Also on that point, being good at Jigglypuff before playing Wario would only help you adapt quicker to Wario's playstyle and help you be familiar with what you need to do when you first pick him up, but this only goes so far until the point where even players who weren't good with Jigglypuff will still be as good as your Wario, because you've both practiced him and have roughly the same skill level as each other.