At the same time, it's a mite irritating to constantly hear OH BUT SHE'S NOT CANON. I mean, we get it. She's not canon, and I might even wish she was, but it's just such a silly, almost spiteful thing to lob back. That's probably the thing that comes up the most that I just really dislike. I'm not gonna deny tons of FeMC fans are a really vocal minority, but that's exactly what they are - a minority. She is definitely not even close to 'shadowing' Makoto when it comes down to it, and I think the way the rest of the expanded stuff tends to shun her proves my point.
But personally, I don't think I'd have enjoyed FeMC as much as I did without the context of having played through Makoto's eyes first. Which I imagine was the point, given that they added FeMC largely to serve as a point of interest for returning players or as an added incentive (in my case) to replay the game through a new perspective. FeMC's contrast may not have suited P3 thematically, necessarily, but the alternative was making a character without its' own identity, a similarly drab, introverted character that would be hard to distinguish from her counterpart and frankly, even harder to justify as a notable or cool addition. FeMC is the way she is because she's meant to be the way she is: optimistic, peppy, maybe even a bit ditzy if you want to play her that way and she's all the more vibrant having seen the world through Makoto. It's why I think it's just a weird criticism to say she contradicts the mood of the game because that contradiction was as deliberate as can be. She's her own character and she'd never be as bold or sunny as she is if she weren't made with Makoto in mind.
And hell, if she's not going to be canon and never will be, what's the harm in having a bit of fun in her own little pocket of the universe regardless of however things are 'supposed' to be?