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I think this has some merit. There is the obvious issue that as the rocket exhaust expands down the nozzle it cools and expands. The ablation will decrease markedly. As with a hybrid rocket the ablation is very sensitive to both.
Also you would add mass to the nozzle that would to a small degree affect its responsiveness and power required to steer the rocket.
But with a 3d printer it should be quite easy to produce a design that is in effect a light weight foam with less density as you go down the nozzle. This should ablate in the desired way and shouldn't add much mass. It would add some mass, and would reduce initial exhaust velocity and temperature; but extra mass should increase low altitude thrust which is desirable. The benefits of a low cost altitude adjusting nozzle is not insignificant.