Why Morin attributes the placing of levers on impedance mismatch? If you use a lever you lift an object more easy. But if the lever becomes very long it is of no use and behaves as springing boards do.
What is the explanation for this?
Morin in his lectures about waves mentions this phenomenon as an example when he explains impedance mismatch but he doesn't analyse more :

Lever: If you try to lift a refrigerator that is placed too far out on a lever, you’re not going to be able to do it. If you jumped on your end, you would just bounce off like on a springboard. You’d keep all of the energy, and none of it would be transmitted. But if you move the refrigerator inward enough, you’ll be able to lift it. However, if you move it in too far (let’s assume it’s a point mass), then you’re back to essentially not being able to lift it, because you’d have to move your end of the lever, say, a mile to lift the refrigerator up by
  a foot. So there is an optimal placement.
D Morin Chapter 4. Transverse Waves on a String section 4.3.2 page 18. 

 A: A lever of every length is going to deform under load. The mechanics are such that the deformation scales with $\ell^3$ for a constant cross section board.
So with double the length, you get double the mechanical advantage, but eight times the deformation. So depending on how stiff the board is to begin with you reach a point of diminishing returns as the board gets longer.
A: Every board is going to bend some when you use it as a lever.
The bending of beams has been studied in engineering for quite awhile, and beam bending has a lot of available literature.
The board is able to deform more if it is longer, because it has more space to bend (or that's one simplified way to picture it).  When you bend a board you're trying to change the orientation of the board, the internal structure resists this, so you can only change the orientation (i.e. angle ) slightly.  When the board is longer, you can picture it as having more distance to get orientation changes, so a small change in angle a bunch of times leads to the board bending with a pretty big angle once you reach the end.
The resistance to the orientation changes has to do with material properties.
This bending would lead to problems if you tried to treat it like a simple lever, as the bending of a long board will change the force on the other end.
Edit to address the "springing" specifically:  If the board is not bent too much, it is in the "elastic region" where it will go back to its original orientation when the applied force is removed.  This elastic behaviour is just like a spring, so sometimes Hookes law is used to describe it ($F = k\Delta x $).
