There are prison ministries that offer Bible classes and discussion to prisoners and it's helpful. Learning how to focus on positive things in life when you are surrounded by pure crap, and have perhaps lived a life of crap is a good thing. Prison is an angry place full of angry people.
The uke is a positive ray of light in life--I use mine to get over grief and many of us attest to its joyful power. So if prisoners could develop a positive attitude from uke playing, I'm for it. Someday they will, in many cases, get out of jail, and they've been abused, fearful, angry, hateful, mean and worst of all bored and let to rust in prison so if they have something that can make them stop being angry and want to join society properly, that's a good thing. Music has been thought to be a spiritual and mental healer since ancient times (King Saul, Hippocrates, just to name a few.)