Super Smash Brothers Ultimate - Looks like even Japan is rather cheesed at Byleth in Smash...

Keep in mind that nothing stops anyone from going to that video and disliking it, Japanese or not. These could just be Western people who also disliked that video for good measure. Comments are off for Nintendo Japan's account, so we have no way of knowing what they really think without digging for it.

" they are 2 complete different genres" I'm having trouble understanding what this means. That Fire Emblem doesn't fit because they are a completely different genere than smash?

Because Smash has characters from RPGs, Life Simulators, Racing games, platformers and even EXERCISE GAMES.