User blog comment:IamJakuhoRaikoben/Chapter 489: Universe One, Review./@comment-25111584-20160615073844/@comment-7418318-20160615075452

I think Acnologia is derisive toward Zeref for personal reasons rather than truly thinking of him as pathetic in terms of abilities.

For example he might once have clashed with Zeref and despite overpowering him, Zeref just wouldn't stay dead. That can be quite frustrating I imagine.

Or it could be that Acnologia does (privately) respect Zeref's power and abilities, but despises his whiny personality, which leads Acno to slur Zeref in any way he can. This one is the one I think most likely.

There's also the possibility that Acnologia was just plainly talking smack. He respects Irene's powers enough to not insult her directly, so he instead chose to insult her through her mentor.