charging is a process of negotiating between charger, cable, and target device. If iPad Pro says 20W is the max amount to receive, that MacBook charger won’t deliver more than 20W to iPad Pro.
It will charge at the maximum that the iPad is designed to handle when you introduce a more powerful charger. That’s probably still at 30W somewhere.
Will the battery health be impacted by this potentially? Possible.
Will you see a measurable impact on your daily life with the iPad? Relatively unlikely in my opinion over the lifetime of the device.