He's not actually ancient - "The Xanathar" is a title, assumed by a number of beholders of varying dispositions over the centuries. Its current bearer (Zushaxx) comes off immature in that book's in-universe commentary because he is (i.e. quite young for the role.)
With that said, I agree with you that not all the jokes landed. I did like the monk one.
I found Tasha's commentary to be much funnier (as befits the witch best known for making people split their sides), and Mordenkainen did a perfect job of portraying the ultra-paranoid batman wizard in his book. I'm looking forward to more of that in MotM, and it sounds like we'll be getting more Tasha there too.