I noticed Tristana will jump regardless if you stun, snare, hook or whatever her at the moment she is about to take off. I got knocked in a wall once by Vayne's condemn but still managed to actually jump. I was briefly stunned when i landed but nowhere near the full stun duration.
This happens with other stuns as well, such as taric's dazzle. If you cast Tristana's rocket jump just before you get hit by the stun you will still jump. Unlike Caitlyn which will still shoot her net when stunned but won't actually get pushed away.
I know some ultimates like ezreal's barrage and lux' laser cannot be interrupted but Tristana's jump isn't an ultimate. And Gnars wallop cannot be interrupted either but the tooltip does actually mention this.