It's sad that the most interesting thing I found about this vote is not the result but those people who doubt the voting system.
Joke entries aside, I don't doubt its legitimacy. But I've always thought 2nd or even 3rd place votes should be weighted like 1st place is. Especially with more slots for characters and music now. Say, five points for 1st, three for 2nd, two for 3rd, and one for the rest.
Mamizou
First impressions are important. I think that with exception, it can be hard to come back if you don't make a good first impact. I remember the reactions, not many liked her at first. She had the double distinction of debuting in a game that wasn't well-received, of which she was thought to be a reason. Her design was seen as not being very appealing, to say the least (ugly was the usual word). It was thought Futatsuiwa from Sado was the best thing about her.
She also suffered from unfair expectation. Unreasonable as it was at the time, fans had concocted expectations of the game and the EX slot and were disappointed. With Mima fever taking hold at the time, it was thought she would show up (I know, it's silly). Or they thought Kasen would make her game debut (it made a little more sense given what we saw of her at the time with the assumption being that she would also be investigating the spirits; ZUN almost thought so, too, but thought it was inappropriate to debut her at that time).
Let's face it, she only started being really cool in Forbidden Scrollery. Hell, she's like an entirely new character. So, it basically took a rewrite, a new design, etc. to get her to where she is. Regardless, I don't think fans have wavered much. The damage is done. Look at how little attention she receives on Pixiv despite having a major role in FS, for example. Now, while I think fans "like" her, like others have said, it's just not enough to move into enough people's favorite characters slot.
Unfortunately, I just don't think many fans care that much when a character gets "better". Ichirin, a favorite of mine from the start, got an origin story in SoPm and was really fun in HM and ULiL, gaining a personality there, but that hasn't helped, either. By the time it happens it's too little, too late. Fans will have moved on by then. Bottom line, it can be hard to shake that first impression.