I'd hate why Pac-Man and Chrom got in, but the characters themselves are just middle-of-the-road bores, whereas the Mii is actively bad, taking up more development time than the average character while being repetitive and grating to the senses, and also far less avoidable online.
Put it this way, would you rather get a lame character that wouldn't require more development time than average and who is just one guy, or an invasive one with no redeeming qualities who requires additional development time for multiple height and weight combinations, all for a moveset that's already functionally in the game twice over, and who is, by its nature, millions of guys?
I understand why Miis would be worse from that perspective, but at least it'll be fun beating the snot out of them, especially since many of them would be embodiments of the Miiverse people. Miis as general video game characters I'm fine with, and I acknowledge their importance and their casual appeal (not enough to make me like them though.) Besides that I wouldn't even play online
that much, since I generally get enjoyment out of Smash from playing with friends. I can't even respect Pac-Man as a general video game character anymore though, he had one game that shaked the video game industry, and that's when it should have ended. However, in true Axl Rose manner he continues and has become nothing but a cheap Kirby knock-off. He's been living off of nothing but his name and his arcade fame for 30 years now, I don't respect anything that lives off of 1 out of 50ish titles.
Plus, while Miis would require quite some development time, Pac-Man is still a 3rd party character, and while Namco Bandai are working on the game it still can't be easy to deal with all ther rights. I mean, it's not like the Pac-Man rights sit with the developers who help with Smash Bros, they're still at Namco's HQ, so they still have to go through the hassle of gaining those rights. Sure it's not Sonic, Snake or Mega Man hard, but still more difficult than dealing with a ordinary Nintendo character.