dressed/done up like a dog’s dinner

The root of this idiom, chiefly used in the UK and Australia, is the phrase ‘a dog’s dinner’ which means- very disorganized, untidy, or messy.
When it becomes the full idiom, to be ‘dressed up’ or ‘done up like a dog’s dinner’ it takes on the meaning of being inappropriately overdressed - garish or tastelessly. To attract attention by wearing formal or decorative clothing when it is not called for.
This phrase is quite similar to ‘a dog’s breakfast’ in that the implication is of something messy and averse, as something socially distasteful or out of place, --an unappealing muddle.