LJN has a reputation for getting a hold of movie rights and them making terrible game adaptations of said games. They did this time and time again but for some reason people kept buying and renting their games despite the fact that these games were rarely anything good. Karate Kid was another brick in the LJN wall of awfulness.
The Karate Kid was such a huge thing in the 80's. Everyone had seen the film and was not obsessed with karate and everyone really really wanted to meet some old Japanese guy that just happened to live in your neighborhood and was a ninja warrior. So when we started to see that there was a video game based on what was one of the most popular movies ever at that point, a lot of us, including me, jumped at the opportunity.
I don't know what we were hoping to receive but I would imagine we had thought that maybe it wouldn't be a boring side scrolling platformer / beat em up that was inferior to the hundreds of games on that market that were already in that category.
They did try to mix it up and have some other stages in-between where you could complete some things from the movie such as catching flied with chopsticks but these were not fun, they were just dumb and incredibly easy
Then we have the problem with how short the game is. There are only 4 stages... That's right, 4 stages in total! Then when you did defeat them all, and I promise anyone other than the worst video game player can beat them as they are crazy easy. The ending is something that took a dev about 10 minutes because the people at LJN apparently have disdain for their customers and the movies that they are representing.
So I wouldn't say that this LJN game is the worst one that they made... That honor goes to Back to the Future in my mind, but this one is close and it isn't always easy to choose which LJN games were the worst, because almost all of them were pretty bad.
Normally I would put a link to a ROM of the game but seriously, you don't want to play this... trust me :)