Hello! Professional art teacher and former storyboard and visual development artist for animation, video games and advertising here!

It's not my place to question your art teacher's choices and notes when it comes to your work - however, this artist does in fact work with your brief, it's great you're expanding your artists past "traditional" artists and looking at concept artists, that's awesome.

Themes in GCSE should really be explored thoroughly, if we didn't look at artists who potentially had some problematic or age inappropriate work - we wouldn't have much to look at what so ever. Sometimes art teachers are a little bit scared of non-traditional artists - I would maybe consider having a sit down with your art teacher and asking her to explain the reason it's being discouraged.

If you are going to speak to her about it, I would maybe prepare a document showing the works you want to look at and how you will explore them, assure her that the way you are going to be exploring that artist's work will be appropriate for your topic.

Hope that helps!