If you want to instance lights in Maya, use Duplicate Special. MtoA will correctly translate those instances to Arnold (as copies, not instances, because Arnold itself doesn’t support light instances).

MtoA doesn’t support the particle instancer for lights.

PS I don’t think Maya supports instancing lights with the particle instancer. The Maya docs say “Do not instance lights; they’ll have no effect in rendering.”, and that’s the behavior I’ve observed with mental ray and the Maya Software renderer. Also, if I leave the viewport set to Viewport 2.0, Maya crashes when I try to use the particle instancer with point or spot lights (and that’s without MtoA loaded).