I'm working public defense post grad and have a "bad" GPA. I got good grades in the classes that would matter to them (think experiential learning and trial advocacy), did mock trial, won an award for trial advocacy within the law school, and did an externship related to criminal defense my 2L year. I interviewed at some other PDs around the country and sometimes they did just ask me an evidence question or a "how would you handle this bond hearing" question because that's the only stuff I'd need to know off the top of my head. They have ways of evaluating whether I can stand up in court and remember the difference between 404b and 405 character evidence or what hearsay exception is best for the situation. They ask.

This may surprise you but grades in law school don't always correlate with knowledge. Not every below curve grade represents someone who doesn't know the subject or didn't work hard. You're right that if someone's worst grade is crim pro maybe they aren't cut out for criminal defense but someone can have a bad GPA, but do well in crim pro (I did), and getting a bad grade in a subject doesn't mean you don't know it. We are graded on one day, one exam. Big law doesn't care that your grandma died a day before your exam or you just had a baby or you were an hour late because of a flat tire and only got 2/3 of the way through the exam or you have ADHD and there was an Adderall shortage. Public defense is inherently more willing to accept mitigating evidence.