If someone is described as being 'dressed like a dog's dinner', it means they are wearing clothes which are inappropriate for the occasion or too formal. This is quite a negative way to describe someone!
The dress code was smart/casual, but Linda came wearing a full-length ball gown! She was dressed like a dog's dinner!
My mum really embarrasses me, she always dresses like a dog's dinner, even when we're just going to the shops!
Look at that guy over there! He's wearing a tuxedo in this old pub. He's dressed like a dog's dinner!
A more polite way to describe someone who is 'dressed like a dog's dinner' is to say that they are 'overdressed'.
That crystal-encrusted dress is very beautiful, but I think you may be a little overdressed for an afternoon at the football.
Five hundred pets and their owners visited the Ichigaya Kamegaoka-Hachiman shrine in Tokyo to celebrate new year and to pray for the animals' health and happiness. The pets received a special blessing during a purification ceremony.