I explained my lackluster grades and MCAT score in those sections. I have a lot of things like B's in very vague/easy/general bio classes, and A's in upper-level bio classes. Or even C's in classes like Art (long story to that, seriously). I wanted to make sure they were aware of that, and that my GPA was not a result of difficult classes that I couldn't handle, but immaturity on my part at the time. I then made sure to reference aspects of my life to show that I've matured. I also explained how that affected my score on the MCAT (little prep in college = difficult to do well on the MCAT without solid studying), and how I chose poorly the materials I used to prep for it.
I don't think there's a problem explaining these things, but only if you can point to other things to show how you've grown/fixed them. Don't just say 'I was lazy in college and got bad grades' without showing how you fixed that, how you've proven to yourself and others that you've turned it around. A friend of mine in his second year at Toledo now was praised quite a bit on his interview for how candid he was about his poor grades (granted, it's a secondary prompt).