Yes, I agree that both Dr Grant and Mrs Norris are speaking truth; that although the tree was bought and charged as a Moor Park, the fruit are not as good as their reputation would suggest. This is enforced by their natures - Mrs Norris is not one to miss out on knowing the value of something she's acquired, and Dr Grant appreciates good food.

So, Sir Thomas produces something valuable and supposedly of good quality and puts it in the care of Mrs Norris. La Norris cares for it, and it turns out to be deficient. Sound familiar?

From Chapter 2: 'Such were the counsels by which Mrs. Norris assisted to form her nieces’ minds; and it is not very wonderful that, with all their promising talents and early information, they should be entirely deficient in the less common acquirements of self–knowledge, generosity and humility.'