Romeo comments that Juliet looks just as beautiful in death as she did in life, with roses in her cheeks and lips. He doesn't realize that the "death" is just a potion that's wearing off. Instead, Why does he figure your Juliet is still so beautiful?

Because as know as in earlier in the play there is a comment "Would a rose by another name still smell just as sweet" meaning that if they looked different or had anything different as appearances they would still be the same lovely person on the inside so he is just saying that she is just as beautiful in death as in life because she is still the nice person she was when she was alive. So now she is no different then when she was alive.