Kim promised to spend the whole day with her mother, but a mission comes up. Drakken is after an experiment which is being transported by train. Unfortunately, Ron is stuck cleaning out his garage (it's his mother's present), so guess who has to accompany Kim?
This is somewhat similar to "Bad Boy" in that a decent idea gets buried in filler. Now, don't get me wrong: it's nice to see the mothers of Team Possible and there's a great gag referencing "Tetris". Unfortunately, the strengths are outweighed by the weaknesses: the action plot becomes dull after a while, and the whole 'Ron is afraid of ________' spiel (this time involving a garden gnome) just smacks of lazy writing (as always).
But these are perfectly forgivable in light of the episode's biggest debit: Mama Lipsky. Some people might find her funny, but I feel that her overly doting nature and naivete at her son's true intentions suck out what little life the episode has. For a much funnier character voiced by Estelle Harris, try Mrs. Potato Head in "Toy Story 2" or Lula on "Dave the Barbarian".
Watching "Mother's Day" is not the best way to spend Mother's Day.