I'm going to give my girlfriend an ultimatum on what to wear
Why would you give your girlfriend an ultimatum on that?
I want us to be a couple who are considered as a pimp and a ho.
Giving an ultimatum on what to wear does not sound like a consenting situation. If she wants to wear it on her own accord it is fine, but you cannot force her to wear anything she does not want to wear.
I want us to be complimented by people when they see us together.
What you are proposing is fine only if there is no ultimatum given to your girlfriend. That is not right to do that to her, she is free to choose what she wants to wear.