Mocks are somewhat stressful, especially for hard working kids, but I don't think we help kids when we act like it's the most traumatic thing ever experienced by human kind. (I don't think you are doing this OP but it does happen sometimes with well meaning parents).
A) they are mocks, not actual GCSEs - the sky would not fall if she underachieved in them.
B) the sky wouldn't ACTUALLY fall even if she underachieved in the real things. They are important, sure, but it helps kids if we retain, and encourage, perspective.
So I think you are entirely right to still inspect her to do some basics of normal hygienic life. If she is accumulating dirty plates and towels, and her room is smelling, that is actually having an impact on others in the family too - OK not a terrible one - but even so.....
I think you are right to comment on it and ask her to sort it. The bin bag threat does sound inflammatory (would you really bin her stuff? Surely you wouldn't. Or is the threat simply to bundle it up so at least it is out of the way - that is not as bad....)
You love her and in the long run it's not worth falling out over. I would go and say, look DD, sorry if I went too far saying the bin bag thing, but I hope you also see life can't revolve just around mocks. I'm here for you and will help as much as I can, but please can you do at least a general tidy and remove dirty stuff?
Make peace and move on. I promise you life and your time with your daughter is too short and precious for drama over these small matters....