Yeah, if we don't count the lore books in BG3 itself, then the only other source of lore from 5e for Loviatar we have comes from her entry in SCAG:
Pain isn't a means to an end for Loviatar's faithful, but an end unto itself. To them, nothing is as transcendent as suffering, and all pain is holy, from the crudest barbarism, to the most sublime torture, to the emotional suffering of the heartbroken or the betrayed.
The pain that one feels is proof of the Lady's attention, and so her faithful are notorious self-flagellants. Pain is also a path to power, in terms of both one's ability to inflict it and one's ability to endure it. A cold, cruel demeanor is considered ideal because it best emulates the Scourge Mistress, and for the same reason her faithful appreciate beauty, cultural refinement, and a certain adeptness at manipulation.
Though temples to Loviatar are rare, her faithful are more numerous than might be expected. Loviatar is the chosen deity of those who inflict pain as a matter of course, including torturers and others who need to break the will of their victims. She is favored by sadists and masochists, and some of her followers form cultish cells of secret adherents. Each of these groups is led by someone who takes pleasure in administering pain and dominating others, supported and backed up by a number of submissive sycophants.
Worshipers of Loviatar rarely gather in numbers except in the more populous cities. When small cadres of faithful operate quietly in such places, few citizens take notice or raise a fuss if they do witness cult activity. The sufferers who endure the lash, however, aren't always willing participants, and Lovatar's cults sometimes operate secret slavery rings, which can draw the attention of the authorities. The open worship of Loviatar and temples clearly dedicated to her are rarely seen except in lands where slavery is an accepted practice.
I was mostly taking note of the liberal use of 'some', 'sometimes' and 'not all' and other such terms throughout it... I don't have copy of the in-game books that BG3 adds to this, but I suspect that this SCAG entry is where they drew most of their background information from, as it's the only current 5e description we have.
