While I don't agree with that '美丽' does not have anything to do with abstract ideal, I don't agree with you, neither.
While 美丽 can be used to describe physical objects, 美丽 in the Constitution does convey a sense of abstract ideal. Because when someone says 'something is 美丽', the statement is already abstract. The statement says nothing about the physical property of the subject, it simply conveys an ideal that may be interpreted differently by each individual.
But certainly this ideal won't be of the Platonic or Aristotelian sort. In this context, it simply means a country that 'everyone finds enjoyable living in', which is indeed an ideal.
edit: grammar