I'm not an admin, so I can't say for sure why "Tiara Mode" was deleted. I guess that a Tiara Mode page was deemed unnecessary because the girls don't change very much when they enter tiara mode. They get a tiara and that's it. What's more, the form isn't even named. Princess Form probably has a page because it's named in the show and has a radical change in appearance.
If it makes you feel better, a Tiara Mode page would be accepted on the Glitter Force Wiki. It doesn't exist yet there.