I'm not really sure there's much value in debating whether " How can anyone legitimately enjoy reading this, honestly?" counts as hate-mail - but telling a creator that their work sucks and everyone hates it is a shitty thing to do regardless. It definitely gets received as hate-mail.
Although having said that, the majority of this guy's posts on lemmy seem to be showing up in random communities and saying he thinks they're pointless or crap.