Why would it be called a mental disorder? If it's a disorder, it's a physical disorder, whose symptoms are the shitty behavior of other people that it "causes".
Or perhaps a mental disorder in those other people, but that's not likely.
I guess I'm thinking it more from the perspective of third parties like insurance. Gender reassignment is generally covered by health insurance and exclusions may be classified as unlawful sex discrimination.
On the other hand, your health insurance could easily create subjective definitions to deny coverage. Because unlike gender dysphoria, beauty is not binary.
With Gender Dysphoria there is only one direction when it comes to re-assignment options, and it is fully documented that this surgery is beneficial for the patient's mental health.
With beauty/ugliness, there are hundreds of possible permutations and no documented precedence to validate that this is a necessary surgery.
Or perhaps a mental disorder in those other people, but that's not likely.