I don’t know anything about manga, but generally speaking, areas of research that have received little attention from academics in the past, but are starting to “trend” among academics and non-academics alike, do tend to be good thesis topics. For one the life has not already been sucked dry out of every idea and data set, so pretty much anything you can do is a result. Plus, people will tend to call your work “exciting” just based on the topic alone.
And what you do your dissertation on isn’t the only thing you teach and research the list of your life. If someone does the first ever academic study of whatever in manga, then it might get them some extra attention (maybe in the popular press?) and help them get that next job. Then they can move on to whatever next hot topic they think will get them published.
Anyhow, that’s my impression. People who have actually finished dissertations can correct me.