Trollface

This meme is known as "trollface". It is used when someone is trolling another person - and indicates that the person that is being trolled fell for the trolling.

It insults the person that it is used on, and so it is funny because insults are funny. It is like saying that the person is taking things too seriously. The art style is sort of cringey, and some people might find that funny as well - it certainly is shocking.