Emperor Georgiou was certainly "morally loose" in very explicit ways, not limited to sexual ones. But, say, Mirror Ezri didn't appear to be.
Bisexuality is just a shooting-fish-in-a-barrel target for this whole "sexual minorities are promiscuous and therefore morally loose in general" trope because the very term establishes that the person absolutely must have at least two partners at all times in order to meet the "happily going steady" standards of decent folks...
Of course, this never happens in the Mirror Universe: none of these "supposedly bisexual" folks appear particularly polyamorous or anything. It's one steady partner at a time, except for the big bosses Georgiou and Kira who'd automatically be depicted with multiple partners even if pronouncedly heterosexual, for that classic crime boss effect.
Timo Saloniemi