According to mcgregor and his coaches, he never specifically prepares for anyone, he only focuses on himself and improving his own abilities rather than game planning for a certain opponent, so that point is null and void.
Yeah? I think they are lying. McGregor usually seems well versed in his opponents' tendencies. And no, the objective fact that McGregor's two worst fights were the two where he took short notice replacements is not null and void.
Furthermore, you're ignoring/dismissing the fact that his opponents in the fights you mentioned had an equally low amount of time to get ready to fight him, so they were at an equal "disadvantage" in that case, the difference being that mcgregor had a full training camp before facing them, so his conditioning was taken care of, whereas mendes and diaz had nowhere near enough time to get into proper shape for a 5 round affair.
I'm not ignoring this at all. This is why I have said repeatedly that Aldo would be taking a risk to fight McGregor under the circumstance of the rematch and that Diaz took a risk taking the fight. Fighting unprepared is a very risky proposition for any fighter. For Mendes and Diaz, as well as Aldo had he risen to the challenge, the risk would have been greater because they'd have no camp at all. For McGregor vs Mednes and Diaz, the risk was lesser because he had a full camp, but non-negligible because the preparation was for a different fighter.
I'm not minimizing the challenges any of these fighters faced. You are.
If you want to speculate on why perhaps those were his worst performances, I'd venture to say that it had more with mendes being a great wrestler and takedown artist, conor's known weaknesses, and in the fight vs diaz he was not prepared for nate's continual onslaught in the face of adversity, culminating in the strikes which caused conor to turn into a panic wrestler.
Mendes' wrestling is very good and would have given McGregor trouble either way, but it's pretty obvious that takedown defense would have been better had he drilled it during his training camp, like he clearly did in preparation for Alvarez. Outside of that one fight, his takedown defense in his UFC fights has been pretty good.
As for Nate, you said it best yourself, "he was not prepared" for the challenges Diaz posed.
I don't know why some MMA fans are so begrudging when it comes to giving McGregor credit when it is due. He definitely stuck his neck out fighting Mendes and Diaz unprepared for their unique stylistic challenges, and it clearly showed. I also give credit to Mendes and and especially Diaz for putting up credible fights under adverse circumstances.