I think they've actually done a good job with Star too, maybe not actually developing her as much but more along the lines of "examining" her. Like showing how all those traits and quirks played off as jokes in the first season are flaws, accounting for her being kind of OP, having her face the stress of growing up without getting everything she wants, etc.
The difference is that Ludo's development makes him more likable and sympathetic, while this season has been (deliberately) going "wow, look at how much of a failure Star really is" over and over.
If anything I think Marco development has been lacking a bit. He got with Jackie and maybe takes karate less seriously knowing his sensei's a loser, but they could probably go deeper.