I am not the best player around town, but to me, missing a mat in 1 must count as a ultimate blunder in a game where the sole objective is to mat the other side. The game I am referencing is this one:
lichess.org/vIswkCrc/black#54 move 28 for white.
I guess it's just a mistake because even after 28. Qd6+ the game is still unbelievebly winning for white. I would consider making a move a winning position equal or equal position losing a blunder. This is just from winning in one move to winning in a couple of moves.
It's not a personal message. They have to program rules that a computer program can evaluate, that work for every game/position. The rules are as good, or very close to as good, as any rules could be.
It's a way to draw your attention to a move.
As the person above says after you look at it, you can consider it as important or unimportant as you wish.