Again, I don't think Xizor was ever really shown as the super badass rival to Vader you think he was, Vaderisnohayden. Far from being "overly done up as 'cool'" I think they might have gone a little far in making him over the top pathetic.
We see the story from Xizor's and Vader's perspectives. From Xizor's view, he is totally winning and making chumps of both the Emperor and Vader. When seeing it from Vader's view, he is getting very jealous, because the Emperor seems to be falling for Xizor's tricks. When we finally learn Palps view, we get the impression he knew what was going on the whole time, and just let it play out, for Vader's benefit or for his own amusement. Sith do have some twisted tastes. So throughout the whole book, Xizor's biggest achievement is making Vader pissed off at him, which honestly isn't hard to do, just ask any of those poor officers who died of a good choking. And making Vader pissed off at you is basically the same thing as putting a pistol in your mouth and pulling the trigger.
Really what we have here is a rottweiler on a leash being held back by his master, while an annoying little pomeranian continually barks and yaps at him. Dumb pomeranian taunts him and runs around him barking and doing flips while the rottweiler's master holds the leash and says "easy boy, stay, stay". Finally, to the pomeranian's horror, the master lets go of the leash and watches with amusement as the rottweiler eats the pomeranian.
Far from being a cool badass villain, I see Xizor as a fool who gets way in over his head. We have these big larger than life characters, Princess Leia, Darth Vader etc, and stupid little Xizor decides he can jump in and play with them. He thinks he can seduce Leia, and he gets a knee to the stones. He thinks he can mess with Luke Skywalker and his palace gets blown up. He thinks he can best Vader, and he gets squished like a bug. It is only ever from Xizor's delusional perspective in the book that we see him as a cool badass villain to rival the best. Everytime something goes bad in his plans, Xizor brushes it off, no big deal, I'll just build another palace etc. His world was crashing around him due to his messing with the most powerful people in the galaxy, and the delusional idiot still thinks he is winning.