This is a biggie, Shmoopers. Sure, "To a Mouse" starts out as a description of a mouse's nest getting torn up by a plough, but by the end, the speaker assures us that it's about something Much Bigger. It's about how all creatures, human or mouse, make careful plans that get all messed up. It's the common fate of all "mortals." Good times, everyone.
The speaker breaks the rhyme scheme in lines 39-42 (rhyming "agley" with "joy") to underline the broken dreams of both "mice and men." Thanks for highlighting the bummer for us, Burnsie.
Far from mocking the hopes and dreams of men by comparing them to those of a mouse, the speaker suggests that all plans and aspirations—from any "fellow-mortal"—are worthy of dignity and respect. So turn those mouse-frowns upside down.