Health Monitor for an expired SSL Certificate?

Can you clarify your question,

Are you asking if one can create a https monitor with cert and key values with some expired cert, what will be the state of the pool members ? Will it be marked up or down ?

The cert and the key values in the https monitors will have no effect. It will have effect only when the backend server actually requires client authentication, in this case the LTM would be the client

So it totally depends how your server is requiring.