I'd say it's a bit too simplistic for TF2's style.
TF2 doesn't avoid things like gradients, dirt, colour variation, shape variations, etc, but it's done comparatively simplistically. I'd suggest making the yellow petals larger, give the stalk a nice curve (perhaps an S-shape), and make each leaf curved down somewhat (upside down U-shapes). Perhaps some better blending of the geometry where the leaves join the stalk.
[Edit] Actually, the leaves look like planes. Rotating them slightly so they don't look "stuck in" would probably do the trick.
Texture wise, I'd suggestion some texture variation on the green (perhaps browner near the base, speckled with a close shade of the green throughout) and adding the "eye" of the sunflower (as seen
here).
These are all pretty small changes. You're off to a good start.
Kep