Jet set babes are seriously into getting big lips right now because rich men are loving it
Generally speaking, people are attracted to facial symmetry, “averageness,” characteristics associated with high social status in their culture, and sexually dimorphic traits characteristic of the sex they’re attracted to. In my research, I haven’t found any scientific commentary on lips in particular, but I’ll give you two hypotheses.
Lips are sexually dimorphic
Male mouths tend to be flatter and thinner while female mouths tend to be thicker and curvier. As such, thick lips are one quality that differentiate women from men, and as such, the more this trait is exaggerated in a person, the more attractive heterosexual men will find that face (to a point).
Lips resemble a vulva
I’ve read a lot of speculation regarding comparisons between the lips and the vulva. Some body language experts maintain that the reason women lick their lips to flirt has less to do with preparation for a kiss and more to do with wetting their lips to make them better resemble an aroused vulva. It’s also been suggested that the appeal of lipstick is to make the lips look more like an aroused vulva, which assumes a deeper red hue when erect and full of blood. When a guy is looking at a thick, red set of lips, he might subconsciously interpret them as a sign of arousal and fertility.
While perceived blowjob skills implied by thick lips may play a role, especially in specific individuals, I don’t think it’s a primary factor in general.