"If you can't do the time, don't do the crime." Dumbass

Is it possible that your interpretation is valid? I mean, sure, but it's not likely. People's choice of phrases often shows their thought process. "If you can't do the time..." is a phrase that's often used to emphasize personal responsibility in a given situation. That, paired with the emphasis but reversal of the key phrase from the headline, strongly implies that this person thinks that it's the failings of those convicted (ie, they did the crime) for their convictions, rather than the police.

The interpretation you bring to the comment is less likely for three reasons. One, a person agreeing with the headline but adding emphasis that the cops need to be held accountable would likely mention the cops somewhere in their comment (only people mentioned are "criminals" ("can't do the time")). Two, people most likely to be critical of cops are likely to want to talk about the systemic failures rather than individual failings (talking about conviction rates for DAs or case closure rates for cops or just plain racism being a large contributing factor). Three, and this is the big one, the most likely comments on Facebook are going to be from those who disagree with whatever is being shown since Facebook feeds you content based on what causes you to engage the most and anger is the most common reason for engagement.