Yes poor acting. Natalie Portman and Kiera Knightly in the Phantom Menace had no vocal variety, Jake Lloyd was just cringy, Hayden Christiansen gave flat performances unless there was literally no dialogue, in which case he shined. Samuel L. Jackson was horribly miscast for Phantom Menace, because he doesn't play a calm, passive type of person well. Only Liam Neeson, Ewan McGregor, and Ian McDarmid did good acting.
And let's not forget about Jar Jar Binks. Don't give me that "sith lord" crud, either. He was a poorly executed character that drug down The Phantom Menace, and is one of the reasons why a potentially-good movie turned out sucky.
Also, the "sith" characters did not get enough screen time to make them shine. The largest example is Darth Maul, who had one of the coolest looks for a sith, especially with his unique lightsaber. Because he didn't get enough screen time, he became one of the most lame Star Wars characters and had to be rectified in the Clone Wars. The best one IMO was General Grevious. The only one pulled off well was Darth Sidious, and that's because Ian McDarmid is a splendid actor and didn't let a bad script get in his way.
If you look at the ideas for the prequels, they'd be amazing if only the script took more risks and the direction was better. But it didn't. That's why they're regarded as horrible.
At least they're very quotable, but that's because Carrie Fisher actually worked as a script doctor for Lucas in order to help the dialogue get better. I fear if she didn't assist him the dialogue could have been worse.
Oh she's a phenomenal actress, she just did a really bad performance in the prequels because George Lucas failed to direct her well.
As a student actor, I can testify that actors rely a TON on the director, otherwise we won't know how to execute the director's vision. If you have a bad director, most likely the film or play you're making is gonna be bad.
I'm not trying to hate on the prequels, I like them. They just have too many problems.