Congratulations to your player in coming up with an imaginative use for the spell. Congratulations to you for enabling it and working out simple, practical mechanics for it.
I personally would have made it a Dexterity (Acrobatics) ability check rather than a saving throw as it is an attempt to keep your balance rather then actually resist the spell and also because more creatures have proficiency in Acrobatics than Dexterity saving throws. I would also consider Strength (Athletics) to stand your ground and give the target the choice.
I would also rule that creatures with other than 2 legs get advantage due to their greater stability.
Is this OP?
Does it break the rule about causing damage? Clearly not.
However, what I think you are asking is: is this overpowered for a cantrip?
My take is probably not and here's why:
- The wizard is giving up their action to possibly knock a single enemy prone. This means they are not using that action for something else like casting a higher level spell or a different cantrip. The chance is 70% for a modifier of 0 on the roll (going down by 5% for each +1).
- If the wizard instead used their action to Help they would guarantee advantage for one of their companions.
- "Fool me once shame on you, fool me twice shame on me". Guess who the creature is targeting next round, you know, while the wizard is concentrating on the Mold Earth cantrip? With a ready action, the caster is concentrating on the spell from their turn up to the trigger point.
- Anyone with a decent Strength (Athletics) modifier can do this as one of their attacks in an Attack action. If they have the same modifier they are better than the wizard (77.25%) and they are even better as the opponent's modifier increases. If they have multiple attacks, they can do this and still attack themselves.
- Don't even get me started on someone with expertise in Strength (Athletics) and the Shield Master feat. Grapple, knock prone with a bonus and they can't get up because their movement is 0.
Readying action requires you to "decide what perceivable circumstance will trigger your reaction." That is perceivable to the character. "When it completes its turn" is perceivable to the player, not the character.
I'm not saying it can't be done, I'm just saying it can't be done with that trigger.
"When it attacks, moves or casts a spell" is a workable trigger but there is the possibility that it does none of those things and the wizard is left with nothing to hang their action on.