1) I absolutely do not agree with that assessment and have no idea how you could say that. They've displayed remarkable range for such a young actor, especially in "Forget Me Not."
2) Blu del Barrio is not a woman and we should not refer to them with feminine pronouns. Blu del Barrio is nonbinary and should be referred to using they/them pronouns.
The problem isn't the actor. They're doing good work with what they're being given.
The problem is that the writers created the character and had about a total of two episodes worth of story for them. This is a recurring problem with modern TV show and movie writing. Writers think it's about economy of plot. But it really comes off as sloppiness. Even laziness. That's why on this show and so many others, character that have an interesting concept behind them become animate plot devices.
Discovery is not supposed to be an ensamble show like other Trek shows. It centers on Burnham. But three seasons in, the show keeps adding more barely explored and utilized characters while other ones there since the beginning sit there and make facial expressions or are glorifed background actors.
I think the prototype example is Worf. In Season 1, the "Klingon marine" was about on the level of the entirety of the Discovery bridge crew whose names many of us have to go to Memory Alpha to look up. He got more exploration in Season 2. By Season 3 he was a fully fleshed out character. 3 seasons in, every single character except Burnham, Saru, Stamets, Tilly, and Culber are at that level. In many ways, the fault is in the concept of the show. With a rent-a-actor in Jason Isaacs in season 1, then a rent-a-captain in Season 2 with Pike, two whole seasons of of character development of one of the leading actors of each season isn't carried over.
Hopefully Season 4 doesn't add any major new characters to ship, and instead focuses on developing the ones it has.