The Cure are thought of as a Goth band... a nice poppy goth band. Though that really is a shit music subculture so it's probably a bit insulting to lump them in with it because of the makeup... lol!
A lot of music style labels are a bit of a nonsense really... often the differences in styles are subtle and obvious only to nerds who are passionate about their particular one.
Ska went on a funny journey... from being poppy up-beat reggae, to a more conventional rock style while retaining the connection (a tenuous one) via the bounciness of that rhythmic foundation... But speeded up. A lot. lol!
I never even understood why goths are fascinated by The Cure. It's not exactly the sort of music you'd be thinking of goths listening to. It just seems out of place except for the make up.
Ska has become a mix of almost everything. When you look at a lot of it, you've got the reggae and rock influences, but a lot of times you'll see a hip hop and blues influence too. The only thing that's stayed 100% constant is the rhythm. But the original stuff still shows through in some of the older bands like Operation Ivy and I hardly think there's a musical genre out there that hasn't evolved over time.