X-Pac was over pretty big in '98 and '99 from what I remember. I honestly think that 'X-Pac heat' was from the fans never forgiving him for turning on Kane.
Nah it was because X-Factor was full of 3 nobodies that no one wanted to see, doing nothing of import, and taking time away from Austin, Stone Cold, the Rock, Jericho, Benoit, Angle, Undertaker, Kane, etc, etc, etc.
The X-Pac heat only really started with X-Factor.
X-Pac really wasn't bad by any stretch. If anything, I'd say he was a solid worker and a great seller. The problem came in how he never was involved with anything meaningful yet constantly got wins despite that.
The interesting thing is that either the Kliq or WWE Talent Relations would toss Waltman out to work with prospective guys. If they were able to keep up and have an interesting match with him, that guy got the green light. I believe this was an anecdote Scott Hall told once.
X-pac was massively over as an underdog face and a heel people wanted to see get what he was due. He was often the one doing DX's run ins and roundhouse kicking a face before doing a crotch chop and running up the ramp with his tongue out. He was an excellent example of a mid carder elevating the whole stable because he could carry a story line. He was small, face and had the right kind of charisma.
'Massively over' is overselling it, imo. He was popular for a bit, but I'd argue part of that was simply just being in D-X.
I don't really agree with your take on 'Shaun', either. Both Shawn and Hunter were great as annoying little shits in the early days of D-X and as a foil to first Bret and then later Austin. D-X started to flounder once they lost Shawn and the world championship (and brought in X-Pac as a Shawn replacement...)
New Age Outlaws and X-Pac both benefited from being in D-X/having Tori around, although they both stuck in/around that faction for far too long. IIRC, Road Dogg and Pac had a match over the 'rights to D-X' which was stupid for a variety of reasons.
Pac has the same problem as Big Show. They're defined by their height and build and there's only a few ways you can use them.
The problem is that Waltman never evolved. About the only thing
Little Bitch did was incorporate the Bronco Buster into his moveset. He was still doing the same stupid fake kung-fu noises/hand motions and spinning heel kicks that were revolutionary in 1993 or whenever, but were stale as fuck in the 2000s.
