If Celestia is the ruler of Equestria, why is just a princess and not a queen?

Those girls are dumb if they think queen is evil

They're not dumb, they are simply going by what they have seen. In most movies the queen is protrayed as evil and nasty, like the evil queen from Snow White, or the White Witch in Narnia (who is considered a queen). It is quite stereotypical, but it isn't fair to call young childen 'dumb' or 'stupid' because you understand something that they do not. I'm sure that you're older then they are, and therfore, knwo better.

Plus. Queen Chrysalis only added to the tradition

What about Queen Elsa? Shes one of the good queens and little girls went crazy about her

MLP was released before Frozen, and Elsa started out as a princess. Even then, Elsa was originally meant to be the villain of the movie.

Sapling132639 is correct. When Frozen first begins, Elsa is indeed a princess, and when Disney first came up with the concept of "Frozen", Elsa was meant to be the villain. The movie was originally called "The Snow Queen". And even as a queen Elsa wasn't picture perfect, as Celestia is often portrayed. Elsa goes crazy at one point, going so far as to almost murder a man in blind rage. You can call this self-defense; I call it temporally losing your mind. Anyways, this isn't to be said that queens are always evil (the queen in "Sleeping Beauty" was very kind-hearted, as was queen Athena in "The Little Mermaid".) However most little girls remember the evil, exciting queen over the kind good queen, so I'm sure they wouldn't take the time to compare and contrast the different types of queens. So just to be on the side of caution, she was named "Princess", because that is how society recognizes queens. Or, at least the little girls that live in society,