Biden has done long form podcasts and speeches, I've listened to them - he's not as sharp as he was but he's fine. He's a terrible public speaker, but he's fine.
Trump is also not a gibbering wreck, but - even ignoring the bleach comment - he cannot remember pertinent details from his briefings (Swan interview), he gives away extremely damaging information about himself on the phone to a reporter who he knows despises him (Woodward). He seems to have little to no control over his own actions. I could be more aggressive and cite the Daily Show's compilation of trump looking lost "Where's Rudy" and confused.
The whole reason why they are pushing the sleepy Joe line is because everything the Donald is accused of, they fling back mirrored against the opponent. Donald Trump, ever since his Nuclear speech, has been accused of being mentally deficient so that's their play.
Edit:
Downvote all you like but please counterargue.