The reason why people got on Matt Mercer when the character Mollymauk was killed rather early in the Season 2 campaign was because the character was a part of the LGBTQ community and thus was a representative for them in this fiction. When the character died, many folks immediately saw it as another example of the
Bury Your Gays trope... wherein it always seems to be the gay, lesbian or queer character that gets killed "for the drama" because they have less "purpose" in the story than the straight ones.
Matt was sympathetic to their stance, because the trope has been in existence for decades and still crops up time and time again in modern entertainment. So he was sorry that the fans (many of whom are queer) had to experience the loss. But he also rightly pointed out that D&D is not like "regular" entertainment, and that yes, indeed, sometimes the dice fall where they may. This was a case of bad luck and the characters not yet at a level to have resurrection magic, which is why the PC actually died (instead of being raised like so many of the characters were in Season 1.)
Now if you are an old school D&D player and/or don't care or roll your eyes at the idea of the 'Bury Your Gays' trope, then I'm sure this whole thing will just make you shake your head. But rest assured that for a
lot of players and fans of Critical Role (many of whom are younger than many of us), the loss of the character, the character's representation in the queer community, and Molly's relevance as a character to the current social climate of the 21st century... was all more important to them than just the "game" of Dungeons & Dragons.