Jubbinaut was correct in pointing out the flaw in the hybrid build, you lose too many stats in either direction to justify it while only really benefitting one skill.
Beyond that, as someone who was an avid AP Tristana player prior to the rework, I can attest that AP Tristana is now really, really bad. AP simply doesn't have the auto attack damage, even with a Nashor's, to make it worthwhile sticking around to pump up her E. Nor does her E-W-R do enough damage now to safely be able to kill most champs. She also lost much of her trade/harass ability when they reworked E. Her old E worked really well with Liandry/Rylai for harass/damage in a fire and forget manner. It now only does mediocre damage unless you pump it up.
Of course if you're taking the time to pump it up for damage then you have to consider the amount of damage that you're losing in those 4/5 auto attacks by not building AD instead. Sure AP has an effective 1.0 ratio, but AD has a 2.2 ratio at maximum stacks-- which would appear at first glance to cover the disparity between the cost of each stat.
tldr; AD is just hands down better right now, because they gutted AP burst/harass.