That is not necessarily an indication that a skill is too powerful, but that it’s the best for what you’re looking to accomplish: ie killing ice heroes
There’s four other elements not covered by it whatsoever, and how often do you see defenses doubling up on a color, especially ice? Almost never. So although it may be the best at accomplishing that, it’s a far cry from being OP, as the balance is built in to the mechanic
If you use this as a definitive answer for meaning too powerful, then I assume you find Ariel too powerful, too? As right now, she’s still the favored healer, so many will level more than one. Same goes for Hel, being the best at mana control. Then there’s Miki, the best for titans.
See what I’m saying here?
All that means [at least to me] is that they’re best at their specific niche, and Mitsuko’s specific niche is killing ice heroes (ala why she’s maxed more than once). But her skill is far from being versatile, as more often than not, it applies to one, maybe two, heroes out of five - therefore, a far cry from OP.
Ursena is a bit more tricky, as she reflects what she’s strong against, which, in turn, makes her more powerful, but I don’t see as many people maxing doubles of her as I do with Mitsy. & My guess, that has more to do with how many other great offensive heroes purple has (where red sorely lacks), and how many offensive ice heroes there are in this game (and deadly ones, at that cough Finley cough)
So yeah, idk if Mitsy was ever considered OP