Normally to me an acquaintance is someone who you don't really know a lot of about, you know them maybe by virtue of a specific social circle but you don't really engage in meaningful or strong bonding interactions with them and your interactions are usually brief moments here and there. Like maybe you see them in the grocery store and it's a hi and by thing. Or you see them at a work function and you talk for a few moments and then move on.
If you're spending quality time with people hopefully by defining it as quality time you know them, you find yourself bonding with them and they aren't just people you would exchange very brief pleasantries with them before moving on.
I guess personally speaking I might find myself a bit hurt if we spent what you would call quality time with me but you merely thought of me as an acquaintance. Obviously that's a personal thing because it's totally your viewpoint on the term friend