Is it ok if I lied to my mom about having a prom date?
What was your mom's reaction when you lied about prom?
She got very happy and surprised.
Even if you were lying to try to make your mom happy, you shouldn't lie to your parents about things. Lying to them is wrong and if they find out it will cause them not to trust you in the future.
I didn't know she would care.
If you didn't think your mom would care, that's even less of a motivation to lie. Lying in general is bad, but lying for no reason is just going to harm your relationship with your parents. Be honest with your parents in the future so you can have a good relationship with them.