It's so fuckin hard to work with people who don't get sarcasm or humour.

The UI guy gives me design (which is a regular design, not something unique)

Me: “wow fresh and unique design (obviously sarcasm).”
Designer: “thanks, I worked on this whole day. Glad you like it.”
Me: “huh...”

Now I feel like a terrible person.

Ya you better be careful, not everyone understands sarcasm, some tend to believe whatever said in front of them

Being sarcastic is a skill which not everyone appreciates.

at least it wasn't the other way around😅
good design, you take him down (sarcasm, as you actually realy like it😊) and then he's mad at you😐...
in case you wanted it better designed you're now on the safe side, you can simply suggest improvements🤷

but i understand the struggle, have had similar situations too and I'm a person with much sarcasm😅

Some people understand sarcasm and also understand that the best way to defuse the situation is to respond like you don't understand it.

Well to be fair, from his perspective, that was kinda douchey. Constructive communication works a lot more than sarcasm