it's absolutely a bad slogan but it's unclear to me who used it during the election cycle. As has been discussed, Dem politicians (outside of AOC discussing a NYPD police matter in June) haven't touched it. I think it's pretty obvious she recognizes she made a mistake, too, and in her latest NYT interview she wouldn't even say "Dems should support Black Lives Matter", she referred to it as "The Movement for Black Lives".
Blaming these unnamed activists and AOC for Dems underperforming is a stretch IMO. The focus group folks sound like they needed an excuse to vote for Trump, and the Democratic Party establishment wants to blame anyone but themselves for underperforming.