According to the rules models on a palanquin can be stomped as they have the Infantry troop type.
The only way this would ever come up in game play though is if an enemy unit with stomp was in base to base contact with your Palanquin character...
From a fluff perspective, I'm torn. The palanquin just hovers. It isn't flying, and the palanquin doesn't make the unit faster, so on the surface there isn't anything that the palanquin does that makes the character more able to evade a stomp. But on the other hand, the caster on a palanquin seems equivalently large to say cavalry, or a Caiman, which can't be stomped.