They hated him because he was kinda right.

There was a lot of disappointment when Byleth was announced, even without all the 'another FE rep' stuff. Because all prior DLC characters were 3rd party, there was this baseless expectation that ALL the DLC reps were gonna be 3rd party. It's basically the same thing as when people say "Spirits can't be fighters, so Shantae will never be playable."

And then when ARMS was announced as the first character of DLC season 2, people were saying that it was also a disappointment because it was a "weak" introduction to the next wave of characters. Even though this may provide hard proof that assist trophies/mii costumes/spirits can become fighters in the same game (emphasis on SAME game).

I honestly think it just boils down to "I only want characters I like, and no characters I don't."