Actually you can't say that (without being ripped apart by some) because doctors feel that is something they can change by surgery. If doctors truly felt it was a mental disorder they would actually treat the mental illness part of it rather than expensive ineffective surgery and a lifetime of meds. Suicide rate among trans people is much higher from what I have heard and surgeries have lifelong issues that continually need surgical repair.
It is more of an issue because of the "beliefs" of the American Psychiatric Association. As with some other "abnormalities," the APA chose to determine that trans-people weren't having psychiatric issues and deemed them as normal. I guess that before they did that, they did believe that trans-people did have mental issues.