Ugh, it's not funny when you have to explain a joke. Especially to a leftist.
Joe Biden can't seem to remember much of anything. His mind is slipping and he's forgetting a lot of things, including how put together and finish coherent sentences. He has dementia, or something similar to it, whether you believe it or not and no amount of ignoring that fact will make it not true.
I am old enough to remember when Kamala Harris called Biden a racist. IF Biden was the one who picked this woman to be his running mate, he certainly forgot that fact. Yeah, it's real smart to pick a VP running mate who called you a racist and a rapist.
So, that all being said and explained why this joke is humorous, is Joe Biden may not have even picked her (which I have my serious doubts) and the joke is that he probably doesn't even know who she is at this point of his ever worsening dementia.
Jesus, it doesn't take much brain matter to understand this is a joke.