Two questions to consider. (1) Are you interested in changing your Dark defense team? (2) How many Dark mythics you got on the team?
If you like the defense team and it’s working well, there’s no reason to hold on to Triandra if you’re not planning to incorporate her somehow. Similarly, if you have two Dark mythic units already and don’t plan to change them out, adding a third won’t change the amount of lift loss reduction because it caps at two mythics. If you are up for altering the Dark team, or don’t have two Dark mythics, then I’d say hold on to her. Otherwise, if everything’s running great and you’re at your max lift loss reduction, I think it’s safe to fodder her.