I used to consistently participate in the "everybody votes" channel on Wii, make Miis for fun, and spend hours just messing around with Miis. Sometimes I pick my Mii in Mario Kart, do Mii related things on my 3ds, and check out other peoples' Miis for fun. I'm not saying that Miis aren't relevant or (in some manor) interesting, I just think they're a really poor choice for a game like Smashbros.
Again, Smashbros is a rare chance to bring together some of the best characters ever created throughout all of gaming history. Miis are simply NOT one of those characters, nor will they ever be.
It's like character creation in Soul Calibur. Yeah, it's fun and everyone loves it. I spent forever making my own custom version of Shredder to play as and I don't even like tmnt. Hell, I don't even really play Soul Calibur! But in the end of the day what makes Soul Calibur interesting is the original character cast, not the half-assed look-alikes that I came up with in 30 minutes. The difference between something like Soul Calibur and Smashbros is that Nintendo has a huge history of original and interesting characters to pick and choose from, which makes Miis look boring and completely uninteresting.
Either way, I'll mess around with Miis once the game comes out. I don't make any final judgements until I've given something a lot of time and consideration. Either way, I still consider Miis to be a relatively boring pick considering how unique an opportunity Smashbros presents to the gaming community.