]Most earnestly did she wish that he might not be too nice, or too observant, if Elizabeth were his object.

I interpreted Anne's meaning to be that she believes that Mr. Elliot has returned to the fold with the express purpose of pursuing her sister. She, in my opinion, is expressing to herself that if this is so, she hopes that ,one, he is not too nice because she knows what he will be getting and would be very sorry for him if he is in truth a really nice man and, two, that he doesn't discern Elizabeth's true character before he accomplishes his goal of 'getting' her because he must either fail at his goal by giving it up as an impossible situation to enter into or go forward into a marriage anyway like a man going to his execution. Just my opinion. Anything to save Anne's honor. :-)