Possibly controversial Stranger Things take:
Mike was actually super in character in Season 5. Wanting to take care of everyone, making El feel better with his stories, being a little whiney, and absolutely glazing Will. If anything his out character aspect of Season 5 was him not playing a central role in the plot, which was unfortunate. The reason his arc felt so frustrating in season 5 was because in Season 3 and 4 he was so violently out of character and then just randomly went back to normal in Season 5. There were obvious changes in his character during those middle seasons (in both normal teenager ways and ways that seemed to go beyond natural development) that were just never explained. It seemed like they were going to make him complex and multifaceted by straying from his original character, but then, by Season 5, he was just Mike again without any interesting reveal or change. It was just that his arc returning him to exactly where he started combined with him playing no important role in the story made him fall exponentially flat.
