It could be a lot, an exhibitionist that might has a lot of sex, a nymphomaniac or nymphophile, a normal prostitute or someone who just enjoys casual sex.
Nymphomaniac is defined as a woman with an uncontrollable sexual desire. I'm assuming nympho is short for that. I don't think I need to explain the latter.
Oshi no Ko is about idols, and idols are *drumroll* puppets. They have no human rights, that includes rights to date or have a family. Also, Japan does not have equal rights, and while male idol might(not will but might) get blasted for dating, for women it's cancellation.
Moreover, since women idols are objectified as holy and pure maidens that don't even know what sex means, if they dare to date they will be called sluts, whores, and so on.