Why did the hipster burn his mouth on coffee?

Answer: He drank it before it was cool

Hipsters are a subculture known for following the latest trends, many of which are ironically retro themed. They can often be found shopping for vintage clothing or hanging out at craft breweries and coffee shops.

Explanation for the kids (and bad puns)

Hipsters pride themselves on following trends before they become mainstream and cool (popular), so the twist in this pun is that the coffee was too hot and didn't become cool (temperature) yet.