It's common here say. I've looked into it. I have found absolutely zero evidence that any cops were killed by "MAGA mob" (your words) nor have I found zero evidence that cops were beaten with blue lives flags.
And it does matter what color line is on the flag because they all mean something different.
What a lot of people did that day is wrong, however contrary to the leftist mob in 2017 and the year long BLM and antifa RIOTS, not one single cop was killed, nor car burned, no buildings set on fire, or even destroyed for that matter during the jan 6th protests.