I've always hated that assumption. Like, think back to Yang's trailer, that titlecard of hers:
"Scathing eyes ask that we be symmetrical, one sided and easily processed. Yet every misshapen spark's unseen beauty is greater than its would be judgement."
Doesn't that SCREAM that Yang is NOT your stereotypical blonde partygirl? People like you look at Yang and default to "Hot blonde chick, likes to party, probably just likes to fuck too" and I think in this case YOUare the scathing eyes, the ones who try to just process Yang as easily as possible. You stereotype her as such when there's clearly something deeper to her, she's not like that in the slightest, and is actually a deep and beautiful person. She might flirt, but I honestly think she's been in very few relationships if any at all.