The title sets the scene: someone has thrown out a bunch of cheap supermarket flowers, the kind you buy to brighten a room or lift your own mood. Tossing them from the windowsill feels like giving up on that effort. The speaker seems worn out by the pressure to look cheerful and keep things pleasant.
The poem itself is tiny, and that helps. It opens with a panicked question, "What do I do?", then answers itself with an order: "Smile." The repetition of "I need to smile" sounds like a person coaching themselves, or repeating what others expect. Then the last line breaks the pattern. It is longer and more honest, asking whether the speaker can simply stop smiling and frown instead.
The mood is quiet, tense, and a little rebellious. There is sadness here, but also a small flicker of self-permission, as if the speaker is testing whether it is okay to feel what they actually feel.