1) Pornography studies can be an aspect of sociology, and it's not uncommon for sociologists to specialize in a narrow area.
2) The study of pornography isn't all that narrow to begin with. It's something that has driven technology to a great extent, in particular media and web functionalities. It's something with thousands of years of history and with literally billions of people involved as producers or consumers. There are probably very few people reading this thread who have never viewed a form of it.
3) A job can have a poor financial outlook even if its subject matter is worthwhile.
Pornography is a constant part of human culture. "Pornography studies" is not meaningfully different from "music studies" or whatever. There is a rich history of both the thing itself and society's interaction with the thing that can be mined for insight and understanding.
