I'm skeptical that it's of any use considering it's only going to be installed by people who specifically want to dislike. It seems more likely that the like/dislike ratio is going to be heavily skewed towards dislikes just by the stated purpose of the plugin and who it appeals to.
I don't think the extension is installed by people that wanna dislike, rather by people that wanna see the dislike count. I used to have it since it's very useful to see if something like a tutorial is a scam.
Also if I remember correctly, the extension was created before youtube completely removed the count, and it scraped the actual value from youtube to add it to the db while the count still was available
