According to the narrator and every character in the book, Mr. Rushworth is a fool. And it is probably true. In fact, Mr. Rushworth appears to be more slow on the uptake than completely foolish. Sir Thomas seems to think him more uneducated than anything else: "Mr. Rushworth was an inferior young man, as ignorant in business as in books, with opinions in general unfixed, and without seeming much aware of it himself" (21.20).
The part about "awareness" is crucial there. Rushworth is ill-informed, ill-educated, and also is unaware and oblivious, which is a personality flaw that appears frequently in this book. He also has another common personality problem: communication issues.
For all his problems speaking coherently, and getting confused easily, and being unable to memorize his lines in the play, Mr. Rushworth isn't a total fool. He notices more than a lot of people in the book give him credit for, even though he has trouble expressing it and even though he notices stuff about a month after everyone else does. He tries, though:
Mr. Rushworth could be silent no longer. "I do not say [Henry] is not gentlemanlike, considering; but you should tell your father he is not above five feet eight, or he will be expecting a well-looking man." (19.32)
Poor Mr. Rushworth seems aware of what's up between Henry and Maria before the wedding, but he just doesn't express it very well. In the end, Mr. Rushworth gets hurt by Maria, gets himself a divorce, and will most likely go bumbling around into another romance in the future.