What is the most user friendly projection mapping software?
I'd hate to answer my own question, but just wanted to share what I found in case anyone else is interested.

PaintingWithLight has a feature rich demo that is very beginner friendly, and only costs a bit over $50 for a full license. Compared to other projection mapping software out there, this is an amazing bargain.

VPT 7 is completely free and feature rich, but not as intuitively user friendly. Luckly it comes with a pdf documentation with lots of pictures to walkthrough someone through every feature.

Dynamapper is a $5 app that also works, but requires some additional hardware (to connect a smartphone or tablet to a projector) and some 3d modeling experience.