Monks have definitely the most distinct visuals among all other classes. Not only all their character animations are unique, but their spells are uniquely jade colored, while other classes have the usual frost/fire/shadow/holy themes, while "physical damage based" classes have almost no visuals, just a little more polished weapon swing traces, compared to past.
Problem is that most people think monk = pandaren. Some even call the class "panda" instead of "monk".
And most people dislike pandaren for how stupid they look.
Also monk tier sets have always looked incredibly stupid and gaudy, making you look like a clown with a rice farmer hat.
Blizz had a unique chance to de-pandafy the class in legion, to add themes from other actual monks that are long established in the game, but they apparently didn't care that much, or simply didn't want to.
As for the gameplay, I'd say most people won't even know whether the class "is lacking compared to legion/wod/pandas" or plays better now or needs specific traits or talents or whatever, simply because people won't even give it a shot as it looks so unappealing to them - again, because they think monk = pandaren.
People like to complain that classes are dull, that they feel slow and that you just press one button over and over - WW monk is the exact opposite.
With 1 sec base GCD and their mastery that literally forces you to press different buttons all the time, the class is the precise opposite of what they complain about, plus they can be almost any race, so no excuses there.