Dressed or displayed in an ostentatiously smart manner.
Why a dog's breakfast is synonymous with mess or muddle and dog's dinner with smartness isn't at all clear. It appears that the two phrases were coined entirely independently of each other.
The first example of 'dog's dinner' that I have found in print is from The Miami News, October 1933:
And on a bus top at a 57th St. traficc halt a youth from the sidewalk called to a young caigaet-smoking lady at the rail: "What you doing sitting there all dressed up like a dog's dinner?"
See also: the dog's bollocks.