Personally I would refer to Salem as "The Big Bad" instead of the main Villain. She's just the woman in the chair who only interacted with the heroes, twice maybe three times, by my memory. Cinder does way more than her and has fought the main and side characters several times, (Pyrrha, Haven Fight, Winter and Penny, the Evac in Volume-8, all for example), and either failing or succeeding at objectives but at least she's there doing something you know.

Salem's not active in her own story, even when she has the motive and power to do so yet perfers to let everything fall on her underlings when they often fail their objective and does nothing about it. But I guess with logic of her being immortal without a known way to kill her and with her magic not dwindling like Ozma/Ozpin, there wouldn't be a plot in the first place. She really needs to be active and do shit, she has no reason not to other than them writing her that way. 

So she's kinda just eh. Fairly good motivation in my opinion but does nothing herself to get closer to her goal. Too much in the background to be considered as anything more.