He wasn't as good as (most versions of) comic Ultron.
He wasn't as good as MCU Loki.
He wasn't as good as he looked like he was going to be from the trailer or first scene with him.
But he still wasn't actually bad, just disappointing, and not what was expected (or advertised).
Ronan, in GotG, was garbage, IMO. Ultron, for all his faults, was still leaps and bounds beyond that guy.
The "weak" power level kind of bothered me, but I could tell they were emphasizing his hive mind and turning him into more of a swarm than a single unstoppable juggernaut, which I can handle.
I think what bothered me the most though, is that, for a brand new form of intelligence, with some odd built-in daddy issues and massive narcissistic genocidal tendencies, there's a ton of psychological stuff they could have explored with him (Vision too), but they really never did more than scratch the surface, and now he's dead. Just a lot of wasted potential there (and damn, we've even got the guy from freaking Jacob's Ladder to work with!). Then again, deep exploration of psychological issues relating to the nature of consciousness is probably a bit too high brow for the kind of audience they're looking to entertain with these movies.
I'm guessing you're too young to remember the greatest Batman of all time