I had my original cats removed due to a noise and had them replaced with 2 aftermarket cats . They aren't a high flow replacement like magna flow or anything but now that its done I have noticed a mpg decrease. Is this most likely because they are new or more restrictive?

BTW its just two pipes of the manifolds with Ok sensors still installed down to the aftermarket converters.

 

The forward OX sensors control fuel injection.
The cats are after the fact and have nothing to do with fuel control unless they are restricting total engine breathing.
The rear sensors only monitor the cat health.
Take a look at the live data for the fuel trim tables. Both short term and long term trim tables for each bank.
A good Scanner is needed to do this, not a code reader grade.
If they are normal there is nothing wrong but if shifted too far it can indicate some issue causing the shift.
Otherwise it's just guessing.
Consider any winter temps setting in and/ or fuel formulation changes that might be the cause of lower observed fuel mileage. Dragging brakes, low tire pressure etc.
Good luck.