Just think about it, you and a couple of friends are watching classic South Park or something edgy. And then, some person comes who's not even a fan of it, demands changes (censorship) or outright turns off the program. Just why do people do this? Why do they ruin everyone's fun?
Seriously, they are. This is by far the most important issue, if they made their asses fatter it would improve viewership. If they made the ponies thicc prudes would complain, which would only draw in even more viewers. Twilight Sparkle, the protagonist of the series has a fat ass, and her teacher Princess Celestia, who's ass is even fatter.
The main reason is uncanny valley. When something looks human but not quite. A perfect example would be a robot trying to mimic being human, it looks unsettling. It's somewhat insulting towards the ponies, you have Twilight Sparkle there a beautiful unicorn and you can't embrace her just because she's not fully in human shape, even though she has all the qualifications to be a sapient-minded
They'd cause a lot of arguments if they tried to play soccer/football, but ultimately not have too many problems.
Somebody will try to put Celestia through one of our horse-based sports, and it might be Celestia herself.
Given their frankly alarming flexibility (and extra limbs for pegasi), I'd say wrestling would be a good fit for canon ponies being OP.
Given that ponies seem pretty flexible, I'd like to see them play Baseball. That would be interesting
They rock at rock-paper-scissors
Polo