What is “OP” to you? Because everyone uses the abbreviation differently. There’s OverPowered, which also means different things to different people. But all characters can be stopped in this game, so there’s that.
Then there’s OPressive. Which arguably, some characters are. But they can fall into this category in several different ways. There’s Hanzo’s problem with being too good at too many things, rendering many other DPS obsolete or pushing down their respective pickrates because he’s always a good choice.
Mei’s problem is that her character by design is frustrating and annoying to fight against. In a way that almost no other character has to deal with, she feels like a character you put into a game when you finally admit you hate your playerbase and you just want them making each other as miserable as possible while you sit back and laugh. And that’s solely because she has some amazing CC/stun without a cooldown.
It boils down to this - things that remove player agency need to be used very sparingly. Most of Overwatch items of that nature are on long cooldowns, are ultimates, and have very short durations. Mei violates this rule with having the slow effect without cooldown, meaning she has infinite CC slow with the possibility of full-on stun. So if she gets into your battle line, you’re going to have a miserable time.
The “teamwork keeps her at bay” thing works perfectly in very high elos, but the lower ranks are more casual players who don’t coordinate or call out as well and she makes life miserable, even if you’re winning. Because sitting there as you rapidly lose control of your character is frustrating as heck, and she specializes in it with no cooldowns to have to manage.
Fighting within her primary attack range is about suicide, which some characters do not have the liberty of fighting outside of it. Others don’t have mobility cooldowns (many of my mains don’t, sadface). Even if you do, some mobility cooldowns don’t have sufficient range, and you might not even have the time to aim in the proper escape direction and activate it because you’ve been slowed down. Even if you do, that slowdown means she still has an opportunity to close that distance again and grab you unless you have a guaranteed out like wall climb. That’s also assuming she didn’t use Ice Wall to prevent your escape anyway.
So she’s an aggravating character to fight against with some wildly bad character design. That’s it.