The poem is a single line, but it says something many people recognise: when we feel sad inside, we often get louder on the outside. The speaker isn't describing happiness. They're pointing out that laughter can be a cover, a way of hiding or drowning out what hurts.
The title helps here. A plaster saint is a statue, smooth and painted, with a calm face that shows nothing of what might be underneath. The title suggests that the laughing person is like this figure: polished and cheerful to look at, but possibly hollow or fragile. Judging by the title, the speaker may be describing themselves, or at least someone they understand well.
The mood is quiet and a little rueful. There's no drama or complaint, just a plain, honest observation. The word "little" makes the confession sound modest, almost embarrassed, as if the speaker is admitting something small that actually weighs a great deal.