I think they didn't make it a round spot for 2 main reasons:
1) Birthmarks are rarely smoothly round, so they made it look more natural.
2) Beauty is a matter of symmetry, and I think that they wanted to diminish how much it would detract from Zazie's
1.) Less about shape and more about color
2.) I don't think her being in make up would have diminished anything...
In a nutshell, my opinion is I should be able to look at a character on screen and without being told know who it is. Again, just how I look at it. I'm sure her performance is fine and I am still looking foreword to seeing it. Probably will be a while though...freaking car repair.