Surprised people are having this much trouble lol.
Well, you have people saying "eazy, git gud" and pro players saying that it's hard even for them.
So some people are lying, or the difficulty and AI are completely random.
But since the online is a hot mess, I would love to know more about the single player difficulty before buying the game...
Single player is pretty hard if you set it to... hard.
As for CPUs in general, they're pretty inconsistent. Sometimes they're punching bags and sometimes they're crazy machines of death and destruction, but they're never mindless frame-perfect reaction computers (which they always were in previous Smash games). Beating them takes some getting used to, but it's not like in previous games where you had to completely change your playstyle when fighting a CPU (and would win pretty much everytime once you did) - the CPUs here are more human-like than ever, including all the inconsistency and stuff. It's actually kinda cool, even when you get used to fighting them it still doesn't feel that far from fighting an actual person.
Also, online is getting a little too much hate imo. It's nowhere near being good but Quickplay is a slight improvement over the For Fun/For Glory modes in Smash 4. Battle Arenas though, that's a huge improvement compared to what was there before. They could still be better, but if you want a decent online experience, that's where it's at.