His power is still the same as ever. Poirier said Conor flash KO'd him in the rematch, but simply never followed up when he was hurt otherwise it would've been another 1st round KO for Conor. As well as that Conor's still the hardest hitting guy he's ever fought. He's also dropped Nate 3x at WW, which nobody else has done in his 4 WW fights since, and KO'd Cowboy at WW. There's no indication his power is gone.
What's changed is that Conor no longer fights like how he used to where there was always a 3rd, 4th, 5th punch lined up so his opponents were constantly getting hit. His power is good and combined with his precision and timing makes it deadlier instead of having Ngannou levels of power, but in reality he was always a pressure/volume fighter. He'd sting a guy and then hit them again immediately while they were already disoriented which is what got him those finishes. People were saying they never knew what hit them because they quite literally didn't see it as he was hitting them while he'd already hurt them. That was his secret.
Nowadays he tries to fight as a hulking bruiser looking for a big punch to put them out as he wants a one punch KO/doesn't want to gas. Instead they get hurt but survive it as there's nothing following up behind it when that's what his level of power needs to be effective. Then he gasses from the extra exertion on bigger punches as well as gets disheartened when fighters absorb his shots.
Like seriously, wtf is this:
He classically Old School Conor baits and catches Dustin clean, making him stumble back. And then he pulls back too instead of pouncing? Of course Dustin survived. Conor doesn't have his old style and finishing intincts.