Is Tristana top viable?

no

Please don't do it

Her leap is not an effective escape tool and so she is vulnerable to ganks as a result. She is targetable during the moment she leaps and is oftentimes killed trying to use it to escape.

And, as is standard with her class, she is weak early.

Taking her top will only paint a target on you that will have no less than 3 enemy champions looking at you like you're food.