No, Trump has always been incredibly stupid. His intellect has only ever convinced people dumber still.

Trump grew up wealthy, surrounded by handlers and supplicants obliged to glaze every statement or utterance. This led this utter dunce to believe his idiotic thoughts are pearls of wisdom offered up to an ungrateful world. Did you know there is a "B" in the word dumb? Have you ever heard of the old-timey word "groceries". Affordability is a hoax. Super intelligence!

He is a spectacularly stupid idiot, and he always has been. This isn't new. Dementia doesn't explain it.