The thing is, it's not like Lapis character is difficult to understand. But even the glimpse and bit we get from her aren't particularly interesting. Lapis may have some mystery surrounding her, but I don't even feel like caring about those.
what to understand?? the only thing we know abt Lapis is that she act certain way (villain) to reach her specific goals (big ?) that have something to do with commoners, that's it, that's the only thing we know abt her, she is still mystery (which is on purpose too by the author), we know nothing about Lapis at all (as of now), so of course it would be harder to understand her than any other characters there.
Natori is the MC we knew everything abt her and her feelings, because this story is from her POV, Diana & Gran are like open book type of characters which is why we already love them, they are open about their feelings (straighforward), the way they act, and how they've shown expression is very easy to read and thus their characters is easy to understand.
and The title here is "The Fed Up Office Lady Wants to Serve the Villainess" so of course there will be a day when we learn anything abt Lapis character, her feelings, her backstory and etc, because she is the MC afterall, and I'm the opposite as you, because personally i like mystery as a whole, so i found Lapis character very interesting, very intriguing and it seems her character is more complex than any characters we already knew so far and if Nekotarou sensei as the author can pull this off, she has the potential to be the greatest character in the story, so i'm excited to learn anything abt her.