The phrase "a dummy of a" is correct and usable in written English.
It is typically used to describe someone or something as foolish or incompetent in a somewhat informal or colloquial manner.
Example: "He was such a dummy of a driver that he forgot to signal when changing lanes."
Alternatives: "a fool of a" or "an idiot of a".
Horsemen play Kokpar, a traditional game between two teams competing to throw a dummy of a goat into a scoring circle, during the first Asian Equestrian Championships.
As a relevant stimulus we used a dummy of a raptor, the European sparrowhawk (Accipiter nisus), placed at the feeding location.
Frank then turned his attention to a dummy of a catalogue he intended to publish, featuring all of his collaborations with the publisher.
The stage will be almost bare – a plastic table, a few microphones and bottles of water, and an altogether stranger object, a dummy of a human head mounted on a mic stand.
In 1919, at the First International Dada Art Fair in Berlin, Heartfield and another Dadaist, Rudolf Schlichter, hung a dummy of a German officer with a pig's face from the ceiling.
A foundation devoted to perpetuating the memory of General Franco is suing a sculptor for allegedly sullying the former dictator's reputation by exhibiting a dummy of the man in a Coca-Cola dispenser.
The pained passion that led one fan to torch a dummy of Modell and hang it from a highway sign in 1995 has diminished.
Around 80% of the material comes from the British Library's vast archives, although there have been some interesting loans from comic book fans – a ventriloquist dummy of Ally Sloper, a Victorian creation who was one of the earliest comic strip characters, for example.
"We went into the chamber of horrors and there was a wax dummy of me in a cage.
Firefighters pushed a life-size dummy of a dead fireman in a wheelchair, his face scarred and burned.
The refuelling valves were filled with 20-year-old cement and the vast bomb-bays typically held a 28lb dummy of a 400 kiloton "bucket of sunshine".
