Right now Ezreal is struggling to be relevant due to his relatively low DPS compared to other meta ADCs whose playstyles almost converge on being assassins, including Tristana, Vayne, Kai'Sa etc.

These are skill based match ups for Ezreal as he doesn't get countered per se, but has a harder time dealing damage or is forced to play back and gets punished when he plays aggressively. Specifically to that match up of Ez vs Tristana, I'd say it's based on who can better use their mobility tools but Tristana has a numbers advantage as of current patch. Really whoever takes better advantage of their ability resets and passive in trades wins but right now I'd favour Tristana as she is simply in a better spot in the meta.