I agree JA can be quite a character assassin but I do not see Edmund as such a one. Can you say what exactly is nasty (mean spirited?) in believing if Mr. Rushworth were not exceedingly rich his family, excepting perhaps Lady Bertram, and particularly his eldest sister would justifiably consider him a “very stupid fellow” (4) just as he does? Mr. R is described by the narrator as a “young man, with not more than common sense” (4) and his performance at Mansfield (6) is rather a confirmation of his light-headedness than not:

Mr. Rushworth was returned with his head full of the subject, and very eager to be improving his own place in the same way; and though not saying much to the purpose, could talk of nothing else.

Mr. Rushworth was eager to assure her ladyship of his acquiescence, and tried to make out something complimentary; but, between his submission to her taste, and his having always intended the same himself, with the superadded objects of professing attention to the comfort of ladies in general, and of insinuating that there was one only whom he was anxious to please, he grew puzzled, and Edmund was glad to put an end to his speech by a proposal of wine. (6)

Edmund kindly ends Mr. Rushworth’s misery (above) and finds a way to honestly compliment him as well:

“But the woods are fine, and there is a stream, which, I dare say, might be made a good deal of. Mr. Rushworth is quite right, I think, in meaning to give it a modern dress, and I have no doubt that it will be all done extremely well.” (6)