I'm no expert, but I believe lppm and lpmm are the same thing, line pairs per millimeter.  Unfortunately there seem to be a couple different graphs that call themselves MTF.  But basically its an all around measurement of 'sharpness' by way of looking at the contrast either at different areas of the lens (center to edge) or at different line densities.

As sharpness decreases, or the line pairs become finer, they become a blur, and you can't measure very much contrast between the white and black levels.  So as the contrast measurement decreases, so does the image quality.

The two graphs I know of work in different ways.  The first plots just one point of the lens (the very center I imagine).  This shows the contrast as a function of your line pair spacing.  This will naturally go down as the line pair frequency increases, even with a 'perfect' lens due to the limiting factor of diffraction.

The other plot shows what lpmm you would get for a certain fixed contrast as you move from the center of the image to the very edge.  This one has a lot of options as far as what the contrast level is defined at and what angle the lines are going, either parallel or perpendicular to the movement you're moving along.  This again tends to decrease as you move toward the right, because correcting for aberrations at the edge of the image is much more difficult than in the center.

Hope this helped!