I actually think this is really clever - exactly because of the negativity some posters are talking about.
Even amongst people who love FatFace, I really doubt it's seen as high fashion or super stylish. I bet it's seen more as nice, comfy, pretty, soft, safe (in a good way), dependable, and similar connotations like that, etc.
So by offering by these sessions, I would bet they're trying to tap into an audience that would feel intimidated by a similar session at John Lewis (which, even though not high fashion itself, has lots of mid-to-higher priced brands and can feel out of people's comfort zone). FatFace is literally a 'comfort zone' for some people, I would bet.
I don't know if they'll necessarily get new customers in, but I think I can see them enticing current / lapsed / occasional customers in to get a session, and those people will end up spending more as a result. (And getting current customer to spend / buy more is always an easier marketing strategy than trying to get first-time customers in).