Miley Cyrus -- It's My Party ... I'll Dress Crazy If I Want To!

Looks like Miley Cyrus' attempt to mature her fashion style and possibly even morph into a Kennedy was short lived ... because Saturday night in Hollywood she returned to her OG.

Miley hit up Penthouse for her 22nd birthday celebration with her new man squeeze Patrick Schwarzenegger in tow.

You may recall ... Miley went pretty conservative this week, especially when they went on a romantic date -- it seemed MC might be switching up her style for Patrick.

Girls just wanna have fun.