When she got the boutique, Diana Prince had been in the world an untold number of years. So it wasn't like American fashion was new to her. Although she did just recently revamp her look to become more sexy. I feel like--once she's disconnected from the other Amazons and she's given up all her magic--Diana fully dives into the life of the modern American woman. And she certainly likes to wear the latest designs--gives the artist something to play around with. And Supergirl is one of her customers--we can assume every new design sent in by the readers is being turned out by Diana's boutique and sold to Linda Danvers privately. Supergirl, being able to drill for gold in seconds, has access to enormous wealth she can offer in return for Diana's services.
And since people so often say this was the Emma Peel Wonder Woman--on THE AVENGERS, it seems like every week Mrs. Peel and Steed are working at some completely new job, with almost no experience to qualify them.