Use mixed metaphors when they serve your purposes. Just make sure you’re using them intentionally.
A metaphor is a phrase like “a man’s home is his castle” or referring to DNA as “the building blocks of life.” It’s a figure of speech that equates one thing—your home or your DNA—with another thing that has properties you want to reference, such as a person having authority within the home like a king does in a castle or connecting DNA molecules together to build something more complex like children do with building blocks.
The Writing on the Wall
Metaphors can also be more complex than just saying that A is B, that life is a highway or that all the world is a stage. For example, saying you can read the writing on the wall is a metaphor for impending doom because it comes from a story in the Old Testament. A blasphemous king named Belshazzar holds a feast, and in the middle of the festivities a hand appears and writes mysterious words on the wall. The king’s magicians and diviners can’t figure out what it means, so the king eventually calls for Daniel, who interprets the writing to mean that Belshazzar has offended God and his days are numbered, and Belshazzar is killed that night. The words foretold his demise, and the phrase “I can see the writing on the wall” became a metaphor for knowing that something bad is coming, especially when other people are oblivious.
When metaphors get overused, people can come to think of them as clichés. Saying someone is “head over heels in love,” for example, uses a phrase that originally meant physically upside down or topsy-turvey to convey that people are so in love they’re discombobulated, but it’s also been used so often that some people consider it to be an overused cliché.