Lucifer isn't just immature this season, though. He is also clueless, buffoonish, selfish, erratic, completely obsessed first with the sinnerman and Cain and now with Chloe,...
And all of those things have been present in the past seasons, but they turned them into extremes while dialling back or completely removing his redeeming qualities.
For example, throughout the last two seasons, and even at the beginning of this season, Lucifer has always clearly cared about others. He cared about Delilah, about various other murder victims and suspects, and about the people around him. Maze, Linda, Chloe, and later Ella, Trixie, even Dan, and finally Charlotte. But now, all that caring has disappeared. He is so obsessed with Chloe that he doesn't care one bit what happens to anybody else.
Back in episode 3.05 he told Charlotte that he felt responsible for what had happened to her and wanted to help her. That was such a big step in his development. But now he doesn't give a damn about her anymore. Same with Linda. She almost died because she got involved with his family, and he was clearly remorseful about that, and worried about her. But now he seems to have forgotten all about that. And finally there is Maze. After their big fight last season and the revelation of how deeply he had hurt her, it looked like Lucifer would finally treat her better. Instead, we're right back to him taking her for granted, disregarding her feelings, and always expecting her to be there for him while never being there for her.