Ablative armor is a bad solution to drag on an aircraft, because the material burns away and creates a rough, un-aerodynamic surface.
You use ablatives on heat shields during atmospheric re-entry, sure, but that's because you don't intend to sustain speed, so aerodynamics don't matter. For a genuinely hypersonic or near-hypersonic aircraft you can't ignore aerodynamics, so you don't even want an ablative hull... Plus, an aircraft whose skin is covered in ablative materials just so it can go fast is an aircraft that has to be extensively serviced after every mission, regardless of whether or not it takes any enemy fire. That's very bad for a combat aircraft.