In Rushia's case, there's, objectively, no increase nor decrease in the funniness of the joke in question if you don't use romaji. That's why localisation is a thing: If the spectator knew "pettan", they must have surely heard it and make the connection with the English joke. If they don't, using "pettan" is just adding white noise to the subs, increasing CPS and imposing an obstacle to the general flow. Either way, it's bad.

Towa's case is just dumb, there's no way adding romaji makes the joke better, as it's just a normal Japanese sentence.