ㄴ데 웃기다

shouldn't be funny, but it is

An adjective plus -ㄴ데 ("although it's…") followed by 웃기다 "be funny" is the standard way of admitting you are laughing at something you probably shouldn't: [슬픈데 웃긴 엔딩], [미안한데 웃긴 여동생], [(민망한데 웃김) 그 정도야?]. The first half names the reason you feel bad — sad, sorry, embarrassing, not even wrong — and 웃기다 overrides it. Most of the lines here are bracketed on-screen caption text a video editor added over a moment, rather than something a person said out loud.

Register: casual; mostly caption text