I read the article before seeing the movie (yeah, it spoiled me), and even with the inaccurate description I was like, "Dude, that's not sexism." And now that I've watched the movie--yeah, Princess Unikitty was obviously part of his creation, because his sister doesn't have Legos, she has Duplos.

Perhaps it's some allegory for the experience of women in the workplace? Or, more likely, it's just a plot trap they fall into when trying to make the demographics work.