Not the biggest Conor fan here but he is a fighter at heart. I've acknowledged this since the Mendes fight, as I remember it was the biggest question mark for me regarding that guy then, and that fight did answer it for me.
His problem when things don't go his way is bravado. Maybe you could call it lack of fight IQ too. He refuses to adapt and pace himself when it's clear he's becoming tired and his opponent isn't going away. He doesn't switch stances when his lead leg is getting chewed up. He tries to fire back when hurt against the cage instead of grabbing a clinch or going for the legs (well, aside from the first Diaz fight). It's like he doesn't want to aknowledge whatever unexpected issues he has to face when they rise, maybe he thinks that if he's in denial about them they will not become real.
He's not the most resilient fighter when it comes to submissions but that's not of huge significance when assessing a fighter's heart. You don't have to look for such specifics, a fighter that doesn't have the heart for it is going to make it way more obvious than that. I believe he has more heart than plenty of fighters that never got called out on it, while not having a particularly extraordinary one either. They're guys that make you question if they have any quit in them whatsoever, he's a clear step behind them, but that doesn't make him a fighter with no heart.