I think it would work well as a MN because I'm sure I'm not the only one who gets confused between Kristen and Kirsten (and Kristy and Kirsty) for that matter. I knew a little Kristen who went by Kiki which was cute.
I love Kiki as a nn for Kristen, that is pretty cute. I went online and saw an interview of actress Kirsten Dunst who voiced Kiki in Kiki's Delivery Service, one of my all time favorite movies. Kirsten used Kiki as a nickname while acting which was so precious.