Translation:You don't look bad in a hat like this.
As a UK native speaker, I think you'd be highly unlikely to use "with a hat" here. In a hat would be far more usual. Compare other items of clothing... "you look good with those trousers"? "you look good with that coat"? For UK English those would both definitely be "in" not with - otherwise it sounds as though you are standing next to the trousers or the coat and looking good with them - and pretty much the same goes for the hats. I agree with ion1122's third example above... if you're using with then you'd do much better including the preposition: "you look good with a hat on".