It means that whoever is being described like this does not have to do much to their appearance to make themselves look beautiful or perfect. Examples: "Sandra is naturally beautiful. She is effortlessly perfect in every way." "Ashley doesn't even wear makeup and yet she is prettier than me. I am jealous of how effortlessly beautiful she is." "Mike's hair is so much smoother than mine. It also stays in place throughout the day. Meanwhile, I spend 2 hours trying to tame my curly hair and it still looks messy. Mike's hair is so effortlessly perfect that it makes me sick!"