Do I need to be playing the VRChat to exhibit?

A VRChat account is required for the exhibition.

You need a VRChat account to exhibit in the VketMall. A VRChat account is different from Steam account.
Please note that you need to have user runk (trust level) not lower than the Visitor, that is at least the New User, to upload your avatars for trying on in stores like Garage.avatar.