I notice most people who rage quit are like.. sheiks who lose to my kirby or meta knight, and it's pretty much exclusively sheiks, not even any other high tiers. My best guess is that it's someone who picked up sheik expecting to body people w/o putting in at least half an ounce of work.
Other times I see people rage quit is when I play what the other player (guessing here ofc) imagines is a high tier or something. I was playing random one time and got diddy, and the other person just SD'd twice from seeing diddy (mind you it looked like i switched to diddly b/c I way playing random). Funny thing is, that was post patch so I don't know why they would just give up. Only once did I ever have someone d/c because I brought in MK after Kirby, so like, they must have left b/c I was winning, or thought this was Brawl still haha
Ah, but if I had to say how often, probably somewhere between every 20-30 matches, it's not often I meet someone who just flat out disconnects or SDs, but it's often enough to put a rough estimate on. If i had to estimate how often someone leaves from the character selector angrily though, then maybe every 5-10, but I don't know how salty they are so I can't really judge.