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.
Best Answer
With both Tristana's Rocket Jump and Ezreal's Arcane Shift, there is a slight delay between when the ability is cast and when the dash/blink starts. In between these moments, crowd control will be applied, but will be canceled/overridden by the movement of the ability. This is particularly potent when dealing with hooks, immobilization, and stuns, and is strangely not documented on either the ability descriptions or the wiki.
While in the middle of the jump, any crowd control which is considered "airborne" (that is, knockups, knockbacks, and pulls) will cancel the jump. Knockups will hold her in place, and knockbacks and pulls will take effect as though she weren't in the jump. Any other crowd control will be applied, but will not affect her ability to move, attack, or use abilities until after the jump.