If itās been recently repainted or replastered, and theyāre only hairline cracks, then theyāre almost certainly just where itās drying out and maybe a little settlement.
I think it should be fine, however, if you see tide marks from water or hear water in your light fitting, I would be worried, as that would indicate that a water tank or pipe in the loft might be leaking.
Structurally, Iād be more worried about cracks in the wall, not the ceiling.