In The Thick Of It, when referring to a woman he finds insane, Malcolm Tucker says:
I've seen the Tom Cruise meme of him going nuts on Oprah, etc. But what is the "brush" reference about? Is this some British-specific humor?
Daft as a brush is a colloquial British phrase meaning crazy or foolish.
I would take Malcolm's phrase as a joke to mean particularly mad as she is as daft as Tom Cruise's favorite brush.