On e621 they use the tag "plump_labia"...over here you guys use "fat_mons". After doing some research I'd say the former is more anatomically accurate. Most of the images that are tagged "fat_mons" the female does not actually have a fat mons...which is the area above the vagina. Instead her outer pussy lips, known as the labia majora, are the part that is actually fat.
Furthermore...in many of the poses characters are in, you can't actually see the mons pubis area, yet they are still tagged fat_mons. like in this image: post #2559945 you can't see that area. You could infer by the overall puffiness of her pussy that she might have a fat mons as well, but that's really just a guess.
Finally, I think more people are looking at and enjoying the plumpness of the outer labia, rather than looking at the mons pubis area.
Probably not going to change a tag that has so many entries, just wanted to have a discussion about this :)