The third movie is total fanservice for people who like Matsu-ken as L, has an incredibly weak plot, tries to make L an action hero, etc etc. Really only worth seeing if you want to drool over the actor for two hours. (I think he’s cute, but his L is too flouncy in comparison to the animated character.)

Most decisions in DN, period, were made to drive the plot. Misa is “spontaneous and not too bright” solely because they needed a character who would make the “shinigami eyes” deal. The Japanese police are occasionally stupid because the plot requires them to be, in a way I can’t describe without major spoilers. Etc, etc, etc.

Most people I know who watch the series find Misa intensely irritating until the five-year timeskip, after which they begin to feel more and more sorry for her. I have some compassion for her as a character because, while she’s annoying and not so bright and, er, a MASS MURDERER, she’s not doing it for glory (unlike Light and Kiyomi Takada). Her family was killed, and she consistently tries to replace them with Light, who is only using her; it’s sad that someone so famous and popular isn’t genuinely loved by anyone. She and Mikami are both pitiable figures.

Reactions from people who read the manga might be different simply based on the fact that they don’t have to hear a grating VA performance. (No insult meant to Misa’s various VAs: “grating” is a fair portrayal.)

You had a Japanese “Loli of the week” a while ago who, IIRC, did Misa cosplay — I remember commenting on her. She really did look like Misa come to life.