I have no idea what Cyborg and Ronda fights you watched, but I'm looking at them right now and those punches are coming from the next state over. They're punches you'd use against a heavy bag where you can just keep your hands at your waists when you around launching haymakers - and that's exactly how Nunes throws them. And yes, in those fights both Cyborg and Ronda were terrible competition. Ronda has always been awful (but her competition has been worse), and Cyborg got drawn into a foolish brawl and got caught.
Besides, what's the "much better competition" you're talking about? Pena wasn't on anyone's radar, but she was 6-2 in the UFC, with losses only to Valentina (something only Nunes had been able to (semi-)legitimately do) and GDR (who turned Nunes into a wrestler). Besides Pena, there was Anderson (3-3 UFC), Spencer (3-3 UFC), Holm (2-4 leading up to the fight), Cyborg (legit), Pennington (come on), Rousey (come ON)...
Nunes's best wins are Cyborg, GDR, and Valentina, all of whom I agree are better than Pena. But all three of those wins have a star next to them in my mind - fluke (Cyborg), lay and pray (GDR), 20 pound weight advantage/dubious decision (Valentina). She's labelled the "GOAT" because she's the best of the two worst divisions in the UFC, but she has never shown much actual talent beyond being a strong, hard-hitting, intimidating fighter.