Oh, they will work. They will be absorbent. Will they be worth your time for the money saved? For me, the answer is a resounding NO

because I am a very slow and amateur seamstress and regular prefolds are very cheap. Real prefolds are made from a more durable fabric (twill) than most T shirt fabric (thin jersey) and last forever. However, it could be a really fun project and if you have some T shirts in fun colors, you could get some cute diapers out of it, so don't let me dissuade you!