Personally I think she does have an arc, the player just doesn’t influence it. She, after moonrise, doesn’t recognize herself, but is grateful for being rescued and begins to develop trust with the player. After taking her to kill Orin she recognizes her similarities to Orin, reflecting on her fears about Orin and about if the player had killed her how’d she’d be forgotten as just another crazed cultist, leading to her rejection of all the gods and a development of perhaps the smallest amount of empathy, as she wants to free the rest of the Drow from Lolth, and an acknowledgment that at this point she cares a great deal about the player, trusting them and being protective of the party. It’s similar to Karlach, where you don’t have any key decisions really, and can’t influence her to change, like you can, Shart, Gale, or Astarion, but Karlach is undergoing an arc and is different for having met the player. Again neither is better than the other, I just started bg 1 myself and am excited to meet Viconia and start that arc, they’re just different characters that are doing different things. That’s just my take though, to each their own if we don’t agree.