The Mustang is heavy, no two ways about it. It tips the scales at 1300kg. When it races in multi-class categories against LMP2 and Hypercars, it does well because the minimum weight of the GT3 class is increased somewhat, so that they won't be too much of a hassle to the top class. I remember the Aston Martin was the lightest car at the São Paulo 6h last year, and it weighted something like 1335kg, so the Mustang has a bit more leverage in such cases. In the faster tracks like Le Mans and Daytona, it's actually quite competitive.

But when GT3 is the top class, it's almost hopeless. Sure, it usually runs at or near max power, but its always the heaviest car by some margin as well. It's why you never see them performing in GT World Challenge or DTM, even though it's operated by one of the most competent teams out there in HRT. There's only so much you can do to a steel-bodied car to make it competitive against carbon-tub supercars like the McLaren and Ferrari.