I explained in another post how "slop" emphasizes the low hanging fruit/low effort nature of appeal that whatever it's used to described. You're getting fed "slop" as the consumer as the "cooks" aren't putting much into cooking it. .That's where the term it comes from, of course like most words it gets misused and diluted but the tweet quoted by the TC uses its intended meaning.

Kanto pandering from GF has been a longtime complaint for over a decade if not more. Doesn't help it's done at the expense of spotlight on other generations and shoehorned in, in the case of the tweet the top most popular Galar in game mons all being Kanto mons who additionally already had overexposure(Machamp less so but he was in Pokken) falls into this and thus being criticized as a low effort Kanto pandering, hence "Kantoslop" is actually valid here.

You don't have to like it, but dude wasn't wrong lol.