No, larger rear tires can cause more understeer. A Fox Mustang is a nose-heavy turd, so if anything you would want larger rubber on the front if it didn't look goofy. Larger tires potentially add more grip, so putting larger rears adds cornering power to the rear, where you're typically not looking for it on a Fox. If you're never going to do track days with the car, don't worry about it. If you are, you'll have to make a decision on looks vs. performance for the wheel & tire combo, or have a dedicated set of mounted track rubber with a square setup.
cheers
Ed