They were all plain/ordinary looking. Those horse teeth! The only ones who had any true good looks were some of the married-in relatives, like Joan Kennedy or Peter Lawford.
Politically, Kennedy was a rock star; I’ll bet he made Bubba look like a piker in comparison. Maybe not every women would sleep with him, but a lot would (married or not).
When it comes to mistresses, I’ll wager the main thing is probably not looks (although ugly women need not apply). Rather, is to allow yourself to be at the man’s beck & call, without almost no regard for your own dignity.