C's are for anyone who wore gloves, put the same design on every nail, didn't bother to dress up their background, ect. Basically anyone who clearly didn't put even 2 seconds of effort in. I also give c's for people who were grossly off theme.
B's are for people who were on theme but didn't didn't put any effort in their background/foreground, or who were slightly off theme but did a good job (put a lot of effort in. But still off topic)
Perfect's are effort+theme. But I don't love them. I recognize the time and effort but they just aren't very striking for me, personally. For instance I like clean more minimalist looks that actually make sense on nails, don't really love the cluttered full landscapes people try to do on a nail even if it is super impressive.