"Sincerity gives wings to power."
Well, I'm glad she was given the crest of Sincerity. Mostly because Sincerity is SUCH a cool word. Sincerity sincerity sincerity sincerity. Also, Joe's Japanese crest was originally Sincerity (which suits his Japanese self very well, I think, but maybe not his English one quite so much - then, his English one was probably MADE with Reliability in mind... eeh, I'm talking about Joe again ^^;;;), so I think of those two as sharing a crest. Eep! That sounds almost romantic! Well, they are good friends, anyway. ^_^;;
Sincerity, as I see it, is being genuine. It's the opposite of hypocrisy - it involves honesty and usually some simplicity of feeling - expressing yourself genuinely. A sincere heart is one that is for real, basically. Sincere words are ones that are from the heart, words that are meant.
At first, Mimi does not seem like a particularly good candidate for such a crest. In fact, when she first received her crest, I was thinking "hohh, sincerity? how are they going to pull this off? good luck getting THAT crest to work!".
See, sincerity is also the opposite of shallowness. There is value and meaning attached to sincerity. And for the first half of the series, Mimi is basically a ditz - concerned with petty matters, rather selfish and spoiled, and shallow, judging things by appearances (her first comment on Angemon was to compliment his hair - after Palmon digivolved she said she liked the way their colours matched - etc)...
When Mimi received her crest, she was proud to receive it but felt that she didn't deserve it... but as she grew up, she developed into someone who *did* deserve it.
It was in the episode "Princess Karaoke" that she really learned about sincerity. In that episode, she was playing at being a princess and getting her own way. When she went out to sing in her old clothes, she was going as herself - saying that she recognised what was important and would give up the fancy life to stay with her friends.
That was sincerity. Part of the song she sang to everyone included the words:
I think that her crest should have reacted in that episode. Actually, when I first saw that, I was fully expecting it to, especially in the ensuing battle - Togemon had the perfect opportunity, but Greymon took over instead, which meant Togemon didn't get to reach Ultimate level for umpteen episodes. -_-.
Mimi's crest takes a while to react, partly because there are few dramatic opportunities for it to do so before then, and partly, I believe, because she takes a little while to mature enough and understand sincerity enough, to enable it to react.
She did have undercurrents of sincerity throughout the series, though. She always said exactly what she thought, and never minded showing her true emotions - she didn't put on a mask in front of the others. Her responses to things, even if they did seem kind of stupid, stemmed from just what she was thinking, and that she would so easily do things from the heart does indicate sincerity.
Mimi sees herself sincerely too - she doesn't put on pretenses, and doesn't bother analysing her feelings or anything like that. Instead, she just accepts herself and sees herself as she is. (A little more on this in the growing up section.)
I would say that Mimi does deserve her crest. In later episodes, when more emphasis is placed on her kindness, trust and friendliness (as opposed to on her complaints and irritability), something about her seems to radiate sincerity - her voice, her smile, the eyes... Just see the picture on the main page to see what I mean.