Diana's hair didn't work with her pearl tiara. It was also too matronly for someone as young as she was (in the early days).
I like dark hair with tiaras. Either long or short, but dark hair allows the tiaras to stand out (and you're less likely to see the support). Martha Louise's hair is short, and her tiaras look fine, regardless. Mathilde's hair is longer, and her tiara looks like an arrowhead. Victoria's hair is long, and she chooses bad tiaras. So it depends more on how the person places and chooses the tiara, than how long the hair is.