The speaker keeps insisting that a smile is small. It's "only" a smile, "a tiny thing," and the repeated "it didn't" lines pile up a list of everything it can't do: feed anyone, protect anyone, heal anyone, or stop the speaker from crying alone in the bathroom. The poem's title, "17 Muscles to Smile," makes the same point. A smile is just a bit of physical effort, so it seems absurd that it could pass for happiness.
What comes through is exhaustion and quiet anger. The speaker seems to be carrying responsibility for others ("protect them when i couldn't") while hiding private pain. The final line turns the list into a question, and it stings because the speaker isn't really asking about muscles. They are asking why nobody looked past the surface. The mood is flat, tired and a little bitter, and the plain lowercase style makes it feel like a confession rather than a performance.