Why do people think hickeys are trashy?

They're just an even more juvenile form of making out in public, which is gross. Have the confidence to not mark yourself. No one cares that you're kissing someone. 

Because it's something only trashy teens do to show off to everybody that they're hooking up, which they do because they know they're trashy and disgusting and that's why even they themselves almost don't believe that someone's willing to hook up with them. So almost not believing it themselves and being totally insecure from being so trashy and disgusting, they feel it necessary to prove to themselves and to everybody with eyes that happens by that they are hooking up. 

The thing is, because they're so socially backward and unself-aware, they think they're impressing everybody as much as they're impressing themselves, but they're not. Instead, other people range from not caring to throwing up in their mouth a little bit by having the idea of two such trashy and disgusting people bumping uglies being jammed into their brain by those hickeys they're ridiculously wearing like it's some medal they won, which I suppose when you're that trashy and disgusting and have certainly never won anything in your whole life, having even another trashy and disgusting person be willing to hookup with you probably feels like what they think winning a medal (or anything at all, really) would feel like if that had ever actually happened to them.

Have you ever seen someone be super proud of something but that's so ridiculous and embarrassing that you just cringe because they are so oblivious that you're actually embarrassed FOR them? Like those idiots who go on Idol or AGT and think they're FANTASTIC but actually TOTALLY SUCK BALLS so bad you cringe and just have to cover your eyes because you're so embarrassed for them! People who give each other hickeys are exactly that kind of idiot.