I'm glad someone asked this question. The Mima fandom is unlike any other in the WTC. While you definitely see pretty devoted fandoms around certain characters, their motivations are very different.
For example, I think the Cirno fandom saw quite a boost in terms of sheer numbers after IOSYS' "Cirno's Perfect Math Classroom" appeared - before then, it was just a lot of people goofing off of ⑨. The major characters in the game have fandoms in large part because of being symbolic of the entire series. There are fringe-character fans as well as nice little islands of fandom around pretty much each of the main players.
But Mima fandom is in a class all its own. She has appeared in so many of the PC-98 games, and had such a fair amount of character development (when compared to other PC-98 bosses, that is) that she more or less represents the entire PC-98 series. She is an iconic character, then, and became a central point of focus for PC-98 fans.
Somewhere along the way, Mima fans developed a reputation for being pretty passionate about their love for the character. She has never appeared in any Windows games, and yet she is arguably the most speculated potential boss whenever a new Windows game is pending. This probably originated from PC-98 fans, but then attained memetic status. Why? Because if there's one thing the WTC will embrace like a fat kid on a Twinkie, it's potential meme material. For example, there was an incident from last year, when ZUN appeared on Ustream, and a bunch of people from /jp/ jumped on to demand Mima's return in the channel's chat. This could very possibly have just been /jp/ trolls doing what comes natural, but even so, it still underlines how viral just the Mima meme is.
I think then that what makes the Mima fanbase the way it is, is that you have a foundation of actual PC-98 fans behind the WTC's eagerness to embrace anything memetic, which has kind of snowballed into what it is today.