Ask your DM, but it is strongly implied that you could not
Disarm is an optional rule, so you first need to know whether your DM will even allow it. If so, it would also be their call whether you could use a stunning strike on a Disarm attack.
The first step in making an attack is:
1. Choose a target. Pick a target within your attack's range: a creature, an object, or a location.
In looking at the game text, Disarm says (emphases mine):
A creature can use a weapon attack to knock a weapon or another item from a target's grasp. The attacker makes an attack roll contested by the target's Strength (Athletics) check or Dexterity (Acrobatics) check. If the attacker wins the contest, the attack causes no damage or other ill effect, but the defender drops the item.
Although it is not explicitly stated, it is strongly implied here that your actual attack targets the opponent's weapon, not the opponent1. For one thing, the opponent's AC never enters the contest, presumably because you are attacking their weapon, not them. If you needed to strike or damage your opponent, that should get harder the better AC they have, but for your Disarm attempt, their AC is irrelevant.
Stunning strike, on the other hand, must be accomplished through an attack on the opponent itself (emphasis mine):
When you hit another creature with a melee weapon attack
Although Disarm does not explicitly tell us what the target of its attack roll is, it seems likely that it is the opponent's weapon. Thus you would not be fulfilling the requirement of stunning strike that you hit the creature itself.
However, you are monk and do have a surfeit of attacks compared to other characters. You might even have one or both hands free. Once you have disarmed the opponent, scoop up their weapon with your free hand as a free object interaction and then carry or throw it2 far from where they can retrieve it; then make another attack to stun a different opponent. You will need to spend more actions, but at least you will interfere with two opponents rather than one.
1Anecdotally, of all the weapon-removal techniques I have practiced (usually against knife, occasionally against handgun, staff, or sword) the disarming is seldom accomplished through damaging the opponent, but much more often through manipulating the weapon itself or their grip on it. Even when strikes are included in the technique, they are strikes to surprise or distract, not damage.
2Or, if you are lucky and their disarmed weapon counts as a ranged monk weapon, go ahead and throw it with your own attack.