"There was never yet fair woman but she made mouths in a glass." What does Shakespeare mean by this quote?

This is spoken by the Fool in Act III, scene ii, at the
height of the storm, after Lear has been kicked out to fend for himself against the
elements.  The Fool, of course, is Lear's devoted follower and goes with
him.

First of all the word "glass" refers to a mirror.  In
Shakespeare's day, mirrors were called glasses.  The line is referring to the vanity of
women.  The Fool is saying that any woman who is beautiful will spend time looking at
herself, primping, and making faces at herself in a mirror to check out her own beauty. 
"Making mouths" is an expression for making faces, or it could mean actually talking to
oneself.  So, the Fool is conjuring up a sort of one-person play of a woman at her
mirror looking at and talking to herself, all in service to contemplating her own
beauty.

In the scene in the play, this line, though the
sort of insightful barb often spoken by the Fool, comes as a bit of a non-sequitor,
since the notion of finding some place to "house" themselves from the storm, has been
the topic of consideration by the Fool just before this line.