If leaf drop appears excessive shrub needs some improvement in cultural conditions rather than being whacked back. Too wet, too dry, nutrient deficiency...

This is when kalmias drop their old leaves.

this is also when they would drop leaves if they're drought stressed. Kalmias don't need much water but they do need more than we get from rain in summer.

They can be pruned, yes, but best done right after bloom in early summer. You can do it now but you'll be cutting off any flower buds for next year.

If it's getting leggy it may not be getting enough sun. Kalmias like a lot of sun, more than rhodies. They can take full sun if they're watered regularly. They're not a shade plant around here.

Actually Kalmias like it quite moist. The native western species is a wetland obligate, seen in bogs etc. G. Schenk even wrote in The Complete Shade Gardener that in our area K. latifolia was best planted near water, it being cooler and more humid there. (Bigleaf hydrangea also loves such situations). I haven't seen anything to indicate he was wrong about that. Many specimens in local plantings are sub-par in appearance. When I see the occasional good-looking one there is some readily observed related factor present like high shade, north-facing aspect, proximity to heavily watered turf etc.