I don't doubt Britney's PERCEIVED beauty due to her sex appeal as expressed through her early music videos, magazine covers, and live performances (e.g. Slave 4 U w/ the python). 
But I'm speaking on her beauty from a purely geometric standpoint. 
Her face is too round and mushy. And not in the attractive Gigi Hadid baby doll-esque way. 
 
She's objectively the pop girl least likely to pass as a model.