If the game will run without the plugin, and the plugin isn't an asset. Then they are okay with it. If it was used to improve workflow or to utilize what the engine already has to offer. Then what you did is great. Also does this plugin have any copyright license? If so, be sure you are following their license information. But in any case, it's a GameJam. They want people to have fun, so I am sure it'll be okay in any case. So as long as this plugin doesn't somehow turn you into a 200 dev team producing a AAA game, ahhahaha