How are 100 Mario Challenges difficulty determined?

Solution 1:

I think Nintendo has never shared the way their algorithm determines a stage's difficulty. However, there is a discussion on reddit where a player shared his/her experience with his own stage.

Below is the conclusion and I suggest you to read her/his full story here.

[...] the difficulty rating system must be taking the typical number of deaths per player into account, rather than just tracking every single life used and dividing by the number of players.

So if 99 people complete your level without losing a single life, and then 1 player loses a thousand lives, in theory the level should still be classed as "Easy" in Super Mario Maker Bookmark.