The game doesn't end after you earn Perfection. You get a special cutscene and some in-game extras, but can carry on for as long as you wish.
For the friendship though, you have no choice than fully befriending everyone, regardless of you appreciations towards them, but you can always lower the heart count after Perfection by pelting them with a slingshot, putting something nasty in the Luau, not talking to them (as long as they aren't at full hearts), or in the case of Penny, answer wrong in her 10-heart event., and eventually bring all your hated ones to zero hearts.
Edit: I think that trick for Penny works for her 8-heart event as well, but just double check to be sure, and make sure not to trigger it before perfection.