Dressing for piano performances is really important - particularly if you will be on a stage. Having watched many performances over the years, could I suggest that if you wish to be taken seriously:-
*don't wear skirts that are knee-length or shorter unless you have seen yourself in a full-length mirror while sitting at the piano.
*don't wear tight clothing for the same reason. You also want to be able to breathe and move comfortably.
*don't wear brief tops or low cut trousers that expose various parts of your torso - the audience wishes to hear a performance without unnecessary distractions, particularly if you are performing a very energetic piece of music.
*don't wear stiletto, or any other kind of high heeled, shoes or sandals if you intend to use the pedals correctly.
*don't wear jewellery that will tinkle or clatter.
Usually darker colours are better - I would suggest a jacket-style top with trousers is the best option, especially if you are not used to wearing formal clothes. If you are planning to play many concerts, then once you have found clothes that you are comfortable in and that look good on the stage - keep them especially for performances. Then every time you get dressed in them, you are helping to prepare for the performance.
Regards,
Elizabeth