To 'not be up to the mark' is a gauge of quality of any kind. A painting, meal, or TV show could be described with this idiom. For example: the beaches on the north coast were never up to the mark of those on the south. 'not up to the task' refers to something or somebody that has to perform an action. It is more often used for people, teams, and machines. For example: I tried to download the file but my old computer just wasn't up to the task. There are times you can choose. You have to decide if you're judging the quality of how a thing is or what it can do.