I think the off-look lies mostly in the mouth, nose, and feet right now. Usually the eyes and eyebrows are one of the most important features in creating a "look" but here the eyes look fairly similar to the character design art (posted somewhere in the comments) and the eyebrows are hidden by the hair.
The feet is pretty simple. She looks likes she's from the boonies without shoes, not some elite Megling Yordle special forces character.
Her nose is too pointy, round, and shiny. It looks akin to some nose stereotypically drawn on goofy nerdy type characters. I know there are way way better examples out there somewhere, but this is the closest representation I could find.
http://fc04.deviantart.net/fs71/i/2010/088/0/0/Dopey_Dwarfs_by_HiddenEden.jpg
And finally, the mouth. The flow of the teeth almost looks like she's missing a tooth at first glance, giving it that final nail in the whatever to give off that very unusual goofy aura, unlike the Tristana we know at all. Don't get me wrong, Tristana is a playful little yordle in my eyes, but she isn't goofy like this.
In the character design art piece (again, somewhere in the comments, look for Rew711's post), she gives off a badass smirk. In this splash art, she is smiling with here teeth really wide as if laughing like "ughuk-ughuk-ughuk~". Perhaps showing more teeth would remedy the current problem. Or making her smile not as wide (vertically, I mean. Making the smile narrower horizontally would only exacerbate the situation).
https://s-media-cache-ak0.pinimg.com/736x/15/e2/df/15e2df67681e1ef9c6864cf3431791f8.jpg