This is not tracked directly by MPF, but there are two simple approaches.
If you want to know the total score accumulated during a mode, you can set a player variable with the current score when the mode starts and then compare that to the score when the mode ends. This will cover every score awarded regardless of where it comes from.
If you want to know the score awarded by a mode specifically, using only the variable_player scores in that mode's config file, the score event includes a "source" argument with the name of the mode that triggered the score.
variable_player:
player_score{source=="some_mode"}:
some_mode_total: change
This snippet will tally up all score changes attributed to the some_mode mode and you can use that variable to display.