You probably need to clean your spark plugs. After flying rich, they might be crudded up. Lean aggressively, from start up to shut down. The exception would be takeoff and climb out. My 172 with a Continental 145 engine burns 6.6gal per hour at 2400 rpm.

12 GPH does seem quite high, however,I would not depend on fuel consumption numbers in the POH. I think they are always lower than the actual fuel consumption.

A couple of possibilites, did you accurately measure your tanks, before takeoff and after landing? Could the gaskets on your fuel caps be worn to the point that fuel is being siphoned out by the slip stream? Is your tach accurate? Tachs typically read low at the high end as they age
Do you have the one piece venturi and the new main jet in your carb? I've heard of increased fuel consumption as a result of that "fix"
Any chance you have a fuel leak in the tanks, the sump valves or the gascolator sump valve?

I have a 150 hp 172 and only see 12 GPH on climb, cruise is anywhere from the high sixes to the low nines depending on my power setting and leaning.

My 172-A with the O-300-D burns around 8.5 GPH running wide open at 5,500 msl.
Your's must have a leak somewhere.Primer,carb,or gas caps would be the places to start.
What about the fuel tank vent behind the left side strut? Could it be improperly positioned and pressurizing the tanks thereby forcing fuel overboard?

That high of a fuel burn rate would foul spark plugs right and left I would think.