Yes, hitting a car at 2 mph can at least crack a taillight. Perhaps shatter it. That's why it's believable that Karen broke her taillight when she hit John's car the next morning.
However, hitting a person is a completely different situation. A parked car is stationary, a human is not. If you hit a person at 2 mph, you will knock the person down, but your taillight would remain undamaged.
The state claims that Karen was reversing at 24 mph. I still doubt that you would shatter a taillight at this speed, although I'm open to a convincing argument (beyond anecdotes). I expected the accident reconstructionist for the commonwealth to explain why it's possible. It's a simple matter of physics: force equals mass times acceleration. But he didn't actually provide any actual scientific basis for his claims.
I know the defense has a couple of PhDs lined up to testify, and I expect that they will be more analytical in their arguments.