I am told the Fandom app allows you do see who it was but I have never confirmed this. Either way, the Fandom app only has some wikis. So I assume it wouldn't be any help for the wikis that aren't on the app.
If it is only one upvote, you can see who it is from the URL. When you click the notification, it should take you to the upvoted post. If the post is a reply, the URL should look something like this: https://domain.fandom.com/f/p/number1/r/number2#number3. The last number is the user's ID. You can see their posts by going to https://domain.fandom.com/f/u/number3.