It's to trained to find Evil Vistanni who are the agents and spies of Strahd.
I'm not sure you can train a tiger to only kill the "evil" vistanti, as opposed to the neutral or good ones. I mean, at the very least, it will also attack and try to kill Ezmerelda if she's with the group. Also, "find" isn't what the tiger is used for - "kill" is what the tiger is used for. Killing random people who might happen to be evil is not a lawful good way to go about things, at least in my opinion. Fighting evil in open battle is one thing, but unleashing a wild animal that
may obey his commands and
may only kill the "right" "bad guys" is entirely another. It's like a comic book villain plot line: it's so convoluted there's no way it would actually work.
Also, maybe some (most?) groups will leave the tiger in its cage. I think most would also try to interrogate the owner of the wagon, which would almost certainly lead to him leaving the town, since he will feel that his disguise is in jeopardy. Either that, or he'd have to convince the PCs that his plan to unleash a tiger on random people wasn't evil and get them in on it.
Either way, it's still very clunky.
There are several backstory pieces that look more and more clunky the more I read them. Why didn't the angel kill Strahd? I mean, "he thought it'd be hopeless" is REALLY stupid. They didn't really have a reason, in my opinion. The whole Amber Temple and the dark powers is clunky (defining the dark powers was a bad idea from the beginning). Every time you read "and then all the good people in location X went mad and killed each other" is really clunky. Most of the book holds together well - the Baba Lysaga thing is a little thin but at least it holds together, for example - but sometimes it just doesn't work. Why would the mad mage make himself mad? Especially since curing his madness suddenly makes him just fine again? That's another example of someone who just goes insane for basically no reason - "because it's hopeless," in this case, except of course when you cure him, in which case he's totally hopeful again. Just like the whole tiger thing: just clunky. Sure it will work if the party is going quickly or they think they haven't seen the big picture yet, but if they actually stopped to think about it, they'd realize it was just nonsense.