Well, how is it going to look? Every item you submit adds something to the impression the committee have of you. So if he mentions you got a C in his class in the letter now not only do you have it on your transcript but its glaringly reinforced in your letter of rec which carries significant weight. At best he then goes on to say how hard you worked. At which point the committee knows that not only did you get a C but that was you at your best.....
I don't think it's a great idea. However, a personal letter from this professor may be better than an impersonal letter from someone who doesn't know you but you got an 'A' with. However, the trick is not to allow yourself to obtain mediocre letters. You ask the person if they feel they could write you a *strong* letter of support. You then give them your resume, supporting materials, AMCAS essay or anything else that can convince them that you really are the next Jonas Salk. I would do that with someone I got an 'A' from....
