One of mine is like that. His favorite combo is a sheepskin half-pad over an Oglvy non-slip baby pad. No saddle slip at all, and very even pressure and heat. I can see improvement in both ride quality and in the sweat pattern on the pad post-ride when I use it.
My other horse goes best in a non-slip baby pad and nothing else.
I don’t think any of this has much to do with whether a person rides well or badly, since I’m sure I suck pretty much equally on both horses. I think you just have to take it case by case.
And yes: if a bona fide saddle fitter tells you your pad is worsening your saddle’s fit, then yeah, I’d definitely believe her - unless and until our own experience proves her wrong.