He's gotten worse over time. I remember during the 2020 cycle seeing video comparisons even between his 2016 and 2020 debates.
I don't exactly enjoy going back and watching him talk, but it's shocking going back and comparing seeing how drastic the change has been.
Here's a bit of the 2016 RNC debate. Trump is still rude and crass. He's not coming off as an intellectual, but he's completing his sentences and making coherent points. He wasn't getting obviously confused like he has been since- talking about airports being involved in the Revolutionary War, claiming Nikki Haley was responsible for January 6th, the famous rant about his uncle being a genius, etc.
For comparison, this is how Trump used to talk, back in the 2016 RNC debate.
He's always been an obnoxious and crass New Yorker, never what you'd call an intellectual or academic. But he used to stay on topic, finish sentences, make coherent points, and generally function. Now he's reduced to somehow shouting and mumbling at the same time, confusing names and years, connecting vague concepts together until he gets applause from the audience.