Trump has been pretty much the same person for a decade. He has always tweeted and spoken like a drunk 4th grader. He lies, he exaggerates and he is apparently very corrupt.
I see no reason to believe there is any relevant cognitive decline like there was with Biden.
But, previously he did want to be reelected so he had a purpose in not doing stupid crap. Now that guard rail is gone and we see his true self. Like I said, it makes no difference whether he has or has not cognitive decline. And frankly, Biden either.