Flanderization
Flanderization is the process through which a fictional character's essential traits are oversimplified and exaggerated to the point where they constitute their entire personality over the course of a serial work.
The term Flanderization was coined by TV Tropes, in reference to Ned Flanders of The Simpsons, who was caricatured over the show's run from a friendly and good-hearted neighbor (among other characteristics, while maintaining his Christian faith) into a dogmatic, evangelical "Bible-thumper".
Flanderization is often associated with and cited as a consequence of "jumping the shark", a term used to demonstrate a creative work's gradual decline in quality which leads to a sudden decline in popularity.