Klingons did blow up a Cardassian star to destroy a shipyard
Actually, that was just a variation on the theme of flare bombs (already seen in "Redemption II"): the star was still intact after the deed. Whether the flare would have damaged a planet, we don't know (and the speed at which it moved would have meant that only the daylight side would have been devastated anyway, for whatever consolation that is).
But I agree that there must have been something "unnatural" about that supernova, or else Romulans would not have stubbornly disbelieved in it even after Spock's warnings. Perhaps it was Klingon retribution for the Romulans blowing up Praxis?