Yep, I've definitely heard that analysis as well, and think it's true. I think his ridiculousness gets memory-holed when he's out of public eye.
Another also related to what you're saying.... polling shows that among people who think they're both old doddering idiots, Biden wins those folks. So its def advantageous for anyone to help portray trump as having a bunch of sr moments.
It's also good just from a typical primary of an incumbent standpoint, as he's effectively incumbent as you mentioned
I'm just not super sure she's in a better position to do that now than she was 4 hours ago. I think she's not.