If it works it's ok. It's not a big deal to spank your kid and if spanking through clothes isn't working as punishment then spanking their bare butt is necessary. Only use this punishment for extreme negative behavior and mix in other punishment such as taking away her cell phone if she has one
Hitting a child on the bare private parts is not ok. Especially in girls. At this age, She understands what you are doing to her. There are other ways to eradicate bad behavior. There are even polls that show that some children who were spanked grew up to have intimacy issues, Especially intimate acts involving the buttocks.
Even though disiplining your child is important, You shouldn't use force with her. That will not help her. You should teach her what is right and what is wrong peacfully, Not forcefully. That will help her. This is not 100 years ago where spanking was normal for people to do.