I just learned a new phrase for saying a woman is a plain Jane. I haven’t confirmed if this is applied to men as well but I wouldn’t see why not. It seems to go like this, since salt is seasoning and seasoning generally makes food more appealing or interesting, then to say someone is without salt, means they’re boring.
Ex. Ela é muito sem sal e não sei o que ele vê nela.
Ex. She is such a plain Jane and I don’t know what he sees in her.
You may also see “sem sal, nem açúcar” which is basically the same, meaning without salt, nor sugar.