It seems that you are missing the point of her job. Her gimmick was that she went with champions and didnt care about the guys she managed and only cared about the gold. Her job was to simply get herself over and that is something that she did very very well. Normally a manager is supposed to be used to get guys over but that wasnt the case with her. Can't really put the girl down for doing her job and not the job that you think she should have been doing.
And you know, that isn't exactly a bad argument. It has a lot of holes in it, but it's not a bad argument. But the fact was that while her gimmick was to put herself over, I'm pretty sure Vince's hope is that Sunny's hotness would attract people to watch the wrestlers that she was managing, and also the feud they were taking part in. Otherwise, Vince would just have Sunny come out by herself, have her wrestle, or just strip for the audience. If had no intention of Sunny giving a rub to someone else, she wouldn't have been a manager, and Vince would find another way to use her. Yes, part of her job was to get herself over, but I'm pretty sure Vince intended for Sunny to put over either her team or the team they were working with. Sunny was used when the wrestler was otherwise bland, and didn't have much sizzle to him.
Now, to contrast, someone like Elizabeth was over, but she made sure that the team she was with got over, as well. As a matter of fact, she never called attention to herself, yet ironically still got a fair amount of attention. As I've already said, without Elizabeth, Randy Savage wouldn't be the multi-time world heavyweight champion we know him as.
And again, it boils down to this question: What has Sunny ever done that was memorable? What has Sunny ever done that will live on as a moment that will live forever in WWE lore. I can name three or four off of the top of my head for Elizabeth. If you can name some moments, more power to you, but I simply don't see that happening.