Movies can absolutely be objectively rated good or bad, all the component pieces can be good or bad, writing, acting, directing, pacing, hell, even lighting, editing and special effects.
The problem is your average movie goer can't tell the difference. Sure, if something is ESPECIALLY bad like the visual effects in the Flash, they'll pick up on that.
Quite more often something can be entirely awful and the reaction is "Well, I had fun..." That doesn't make it "good".