The poem is a single line, and it turns a familiar idea upside down. We usually say that things look better in the daylight: that worries shrink after a night's sleep, and that the morning brings clarity and hope. Here the speaker says the opposite, or at least offers a warning: not everything looks better in the daylight.
The word "not" does a lot of work. It doesn't claim that daylight is cruel, only that it isn't always kind. Light shows things as they are, and sometimes what it shows is a flaw, a loss, or a truth that darkness had hidden. The speaker seems weary, perhaps disillusioned, and judging by the title, "Broken Princess," maybe feeling damaged or exposed. The poem doesn't spell this out, so that reading is an inference.
The mood is quiet, wry and a little sad. With so few words, the silence around the line makes it feel heavy, like a thought left hanging.