Like, a personal apron? An apron for business?
I guess 「自前」 used here is close to BYO service, where you pay some charges at the restaurant and they allow you to "bring your own bottles" and enjoy with their meals the restaurant serves.
Probably 高橋みなみ brought her own apron to enjoy cooking which was not what the sponsor or the show prepared for the TV program. Probably an idol bringing her personal belongings to reveal her private life a little bit is important in the program.