I'm asexual, so not allot of things are hot to me. The expulsion of air is one of the things living things do. Natural functions like that are usually never anything to bat an eye at or get worked up over. So it's really not unless you have that specific fetish.
Its not hot...but its not disgusting either. Its just a burp, its natural. If she's beautiful herself, her burping wouldn't change my mind on her beauty, nor would my mind change on an "ugly" girl.