The Mega redeems it a little bit because I think it's a creative idea, and I like the added purple.
I actually have a bit of a story for why I dislike Mawile, so I guess I'll share. (Sorry for the text wall.)
During Gen 6, I was a dedicated mono Fairy user and Karenfag (Don't worry, I was a shitty player). Fairy mono had a bad reputation then for two reasons: every (low-ladder) team was the exact same (BD Azumarill, paraflinch Togekiss, Scarf Gardevoir, M-Mawile, dual screens Klefki, Clefable), and it had already hated mons like Mawile, Togekiss, and Clefable. As a Karenfag does, I tried to stray from the mold and use some of my favorites/"unique" sets, but I still got a lot of "ugh, fucking fairy" comments whenever I battled people. Out of all the Fairy-types I didn't use, I disliked Mawile the most because it got a huge surge in popularity thanks to its overbearing viability. Mr. Mime had been my favorite even before it became a Fairy-type (I'm so happy it did), so it was annoying seeing "Mawile is my favorite Pokémon and I love it so much!" when I doubt a lot of people even remembered it existed before it became actually useful. After a while, it got banned from the tier. I was one to keep an eye on usage statistics, and I noticed that Fairy's usage plummeted from high-ish mid/mid. I think it was once in the bottom 5 of all types. And of course, I still heard things like "so glad it got banned, fairy is so op" and still got shit because I used it, even though it was such a rare type and Mawile was the only thing about it that was OP. I loved most Fairy-types and the concept of the type itself, and I hated seeing so many people associate it with one Pokémon I even didn't like all that much just because it was strong. I'd probably think Mawile is okay if not for all this. It's not a bad design, though it's not up my alley to begin with.
TL;DR Hipsterism + it gave Fairy a bad name in the tier I most followed.
It's not the most mature reason, I know.