In a midsized city, grades will matter a bit. You won't need to be top of the class, but the bigger the metro area gets, the more competitive these jobs are.
If you're looking in a metro area (I'd say midsized city would count), your grades will be a factor, as well as your school, and your experience (if you've never worked for a DA's office before or haven't done any trial work via internships or summer jobs), you may have a hard time. In that case, you may want to cast a wider net and try to get into the solicitor's office (if your state has this sort of position). You'll handle only misdemeanors and traffic stuff, but you'll be in court everyday. Quite a few of the ADAs in the office I'm working for now started in the solicitor's office, and moved up the ranks after they'd gotten more court experience.
If you're looking in smaller cities or rural counties, grades will matter less; they'll still look at experience, but the positions aren't as competitive, and things will turn more on whether they like you when you interview.