Why is the amount left in my ore patches still doing down?

This mod only refills ore patches when any given tile is about to run out. It is therefore perfectly normal to see the amount on each tile going down as you mine it.

With this mod, does the richness of ore patches matter?

For non-infinite resources like iron and copper, richness doesn't matter.
For infinite resources like oil, this mod prevents the yield from going down. Therefore, the initial yield value does matter.