For most of its length, "lie" reads like a simple report of a good day. The speaker says everything was good, that they smiled a lot, that they didn't fake it or dwell on sad things. The language is plain and short, almost like a list of boxes being ticked. It ends with a flat, confident "yes, i was happy."
Then comes the turn: "but, no. / i lied." Everything before it suddenly looks different. The speaker has been insisting so firmly on their happiness that the insistence itself starts to seem suspicious. Even the odd line "i didn't try hard to be delight" now sounds like someone trying very hard.
The mood is quiet and a little heartbreaking. Nothing dramatic is described, and the speaker never says what the real feeling is. That silence leaves the reader with the sense of someone who has been pretending, maybe even to themselves, and who finally can't keep it up.