He is a guy who ties people to the front of his boat to sacrifice them. He looks really young and always has favorable winds (probably due to the blood magic). He is smart, cunning, has valyrian steel armor, and a horn that takes over dragons.
He is a badass in the books, and the show writers turned him into a hillbilly, dick joke making punch line.

He is much more OTT-ridiculous, but has a charm to him that neither Pilou nor the Double Ds have been able to capture.

I can't tell you what he's really truly like in the books, because I gave up on the Iron Island Kingsmoot crap about 851 pages in. Maybe someone else can tell what happened in the final 23 chapters?
Seventy-seven courses and a regicide, never a wedding like it!

Good actor, debatable casting.

Blame the actor too. Actually, blame whoever did the casting. He's about as scary as a Girl Scout selling cookies.
Wait..What?

Is he as much of a buffoon in the books as he is on the show?
It's not the time devoted to developing the character as much as the central flaw in the characterization. Not blaming th actor, but the writers.
As of today, I have read
31
pages of Game of Thrones

Hear, hear.

I think they've given him as much screen time as he needs. I mean, what is there to him, he's a jerk with a fleet!
As is, they've established that he's taken the Iron Islands away from the rightful queen, he's a horrible person, and that he failed to ally with Danerys first. The books devoted about 1400 pages to his rise, and it felt right when the show cut the whole mess down to ten minutes of film.
Seventy-seven courses and a regicide, never a wedding like it!

While I agree with that some whatI think it would have been best to just have a couple more mentions of him. In the first season he is mentioned when Tyrion talks to Theon (well roasts himthe complains later that Theon roasted him..I digress!).
If they had talked Euron up some more with these stories I feel like it would have worked out. Instead we have an underdeveloped hillbilly who is supposed to be a huge badass.

Well it wouldn't have taken much. They could have had a couple of quick scenes of him pillaging on the high seas and then had the Greyjoys mention him. It just feels too late for a new villain.

He kind of shows up late in the books as well. The show probably didn't have time to develop his character, so they picked the Kingsmoot which he had to be at.