An absolute shambles. If something is a dog's dinner, it looks a mess or you've cocked it up so badly it needs doing again. You'll hear it in "made a dog's dinner of it" for bungled jobs. Slight curveball, "dressed up like a dog's dinner" can mean someone's gone too posh and a bit try-hard.
"Tried to tile the bathroom myself and made a dog's dinner of it, wonky rows, grout everywhere, and the missus fuming."