The cartoonish Tristana in her splash doesn't suit the backround art style
I think that is the main problem with the splash :/ The updated splash Trist got a couple of months ago had quite a lot of depth both on Tristana and the backround she was in.
Now? We have this weird case where we have a wonderfully drawn, "serious" backround and for some reason popping out of it is a weird cartoon character. She doesn't fit.
I realise the whole point of the update for Trist is to emphasise how carefree and I guess "comical" her character is but as others have said, she really does look like someone from a Saturday morning kids' cartoon show that is also somewhat lacking mentally.
I'm not sure how you're meant to take this new Tristana seriously or generally find her likeable. I'm not a big fan of her so I am not too invested in how much I dislike this, but I really hope "Yordle champs are cartoony :D" isn't Riot's policy going forward. Give me old Tristana any day