So we aren't talking about how M-mence looks like a croissant, which is nice.
A removed post brought up something that I found telling, and that is the fact that people haven't been mindlessly kvetching about how silly M-mence is; posters of all skill levels have been trying to come up with something, anything that can take a hit from M-Mence and retaliate sufficiently to remove it. I tried SashZam, with the goal of switching it in, Tracing Intimidate, and hitting with HP Ice, which OHKOs after SR IIRC. The problem is the speed tie post M-Evo and the proba/possibility that it's SubDD, in which case you rely on winning a speed tie, which is neither feasible nor desirable, and SashZam is pretty feh regardless.
I saw Scarf Noivern as a possible counter for SubDD a few pages back, but this is very obviously a suboptimal set, to put it mildly. Likewise, Rotom-W requires an hilariously unorthodox EV spread to do anything to M-Mence which renders Rotom-W unable to maintain its mixed defensive capabilities.
Porygon 2 is not completely ass in OU, but it requires a Balanced/Stall team to work effectively, which is undesirable because we really want to keep Offense, Balanced, and Stall at least somewhat viable. Regardless, P2 doesn't work vs. M-Mence unless it can Trace base Mence's Intimidate, so you have to preserve P2 (at full health!) until your opponent chooses to use M-Mence.
I'd talk about Scarf Greninja, but I'd gently remind everyone that this set was considered warm, dirty, stinky stanky tootie before M-Mence.