I doubt it was crema mexicana, which is just sour cream with a (sometimes) runnier texture.
Well, it could be some sort of weird spinoff, like a former Azteca stole the recipies and went off on their own. That actually happened with another Mexican place around here (though it was back in the 80’s, and the spinoff long since died out).
Whatever it is, they serve a dish that is exactly as you describe at a Mexican chain called Plaza Azteca here in the Philly area. Next time I go in I’ll ask.
That’s what I was thinking. The fact that it is described not just as “sweet,” but as “sweet,” leads me to believe it’s not at all plain ol’ Mexican crema (or any of its gazillion variants.) As for pastry cream, I can’t imagine why tortillas would be served with pastry cream. The Miracle Whip suggestion boggles my mind, but that seems to be the best guess so far, given the description and the fact that, apparently, this actually is a thing at some restaurants.
My guess is it’s a yogurt base (possibly sour cream) a little lime, cummin, jalapeño, something sweet. It has a wonderful balance of sweet and tart with a pleasant bite of chile at the end.
Upon further research, it appears that whoever suggested Miracle Whio up thread might be right. A number of recipes trying to replicate tge Plaza Azteca white salsa call for tge dreaded Miracle Whip
I honestly couldn’t get past the Miracle Whip to taste anything else. And, as an aside, I hope that my Miracle Whip-loving late boyfriend is looking down on this thread and smiling. 
ETA: It just occurred to me that our Azteca doesn’t serve it all the time. Too bad as I’m sure that he would have enjoyed it the few times that he ate there.