I will agree that her pre 6 ganks pale in comparison to her post 6 ganks. Many times when I like to play a character in the jungle, someone will say "he's a better top" (or mid). My point is that a character being better in 1 role than another isn't what you have to weigh when you are deciding where to play them. You have to think about, is Diana better mid than whatever other character you team would have played mid, and is she a better jungle than whatever character your team would have played in the jungle. I think she is capable in both roles (in my humble 1090 Elo opinion) so you can make a choice based on how you want to play and what would be good for your team.
Hope that helps.