Conor is not the same fighter he once was due to a few factors
First, simply because when he was coming up through the ranks and had to literally earn contender matchups and title shots, he had the hunger to ''become somebody'' in the mma world.
Unless the person is absolutely obsessed with the sport like GSP was or has a seemingly unbreakable mindset like Jon Jones, it's very hard to win win and win without losing here and there.
Second, because of the inactivity. He himself mentioned that as a reason for this loss. Getting the activity back is very important.
But lastly, and this is the most important one for me, his fighting style simply was perfect for the featherweight division. He can do very well at lightweight and even at welterweight under the right set of circumstances, but maybe not to the extent where he was doing at featherweight. At lightweight guys have either the same speed as him or the same knockout power as him, so it can be harder to take shots or to maintain a high pace for long. At FW he was often the bigger, stronger man.
Mcgregor is a fast starter who used to end fights early, a little bit like Rumble Johnson.