Will a webcam cover crack my MacBook or laptop screen?

It can if the cover is too thick. Multiple threads on r/macbookpro and r/macbookair describe sliders acting as pressure points and cracking displays when the lid closes. The community consensus is to choose covers under roughly 0.03 inches (0.7–0.8mm) and to verify thickness against your laptop's bezel lip before sticking anything down. Notch-equipped MacBooks (2021 and later) are especially sensitive.