I am searching for the title of a movie. I'm afraid I don't know much except how it was described to me. According to my friend, it's based on a girl who travels through mirrors to other realities or the like trying to find a cure for her sick mother. Somehow involves tall clowns and an evil witch. May also include breaking a mirror and cutting a string.

The Hugga Bunch (1985) (TV) (http://www.imdb.com/title/tt0173915/) ?

I'm pretty sure it's not this one. My friend made no mention of creepy dolls.

I'm fairly certain it is "The Hugga Bunch". The "tall clowns" are probably the dolls. But what you described is The Hugga Bunch exactly.

Try the movie MirrorMask.

MIRRORMASK is the story of Helena, who works for the family circus, and wishes--quite ironically--that she could run away and join real life. But such is not to be the case, as she finds herself on a strange journey into the Dark Lands, a fantastic landscape filled with giants, Monkeybirds and dangerous sphinxes. Helena searches for the Mirrormask, an object of enormous power that is her only hope of escaping the Dark Lands, waking the Queen of Light and returning home.