Sometimes when running a simulation I randomly get one amazing creature that I'd like to keep, what I'm not sure about is whether that creature is affected by the mutation rate when I check the box for "keep best creatures for the next generation"

Ideally I'd like it not to be, is that possible?

When the "Keep best creatures for the next generation" option is selected, the chromosome / brain of the best creature is copied over to the next generation without any mutation. It might even be the best two creatures - I don't quite remember that right now. But they are definitely not altered in any way.