Try telling that the the planet Venus. It’s the poster-child of runaway global warming due to the “greenhouse effect”.
Clouds do reflect some of the sun’s rays. But what does reach the surface is reflected by the ground not as light, but as infrared heat. This is trapped very effectively by clouds. And it turns out high cirrus clouds (and contrails) are more effective at trapping heat than low altitude clouds. As a result, less heat is lost (especially at night), resulting in a net increase in temperature.