As a Florida resident, I can tell you that Florida certification is a red flag regarding the trans community (as is Texas certification).
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All she can do is make general statements, as in a clinical setting, it is a dialogue between the clinician and the patient. And while some, perhaps many patients share the same issues, each patient must be assessed on their own terms. We have no idea who her patients are, whether they are satisfied, whether or not she feels she was a success or a failure.
It is a YouTube channel that promotes her, and that is surely the key point.
The same could be said of religious fanaticism or obsession with acquiring wealth and power. It's always been the case that what gets defined as abnormal behaviour depends on prevailing views in society about what behaviours are normal and acceptable.
To be fair to Dr Z, I don't think she's saying that an interest in crossdressing is inherently a sign of a mental disorder, only that it may be a manifestation in some people. That would apply also to the other behaviours mentioned above.
The general criteria for defining mental disorder these days is mainly about whether it impairs normal functioning in a way that is likely to harm the person or others. https://en.wikipedia.org/wiki/Mental_disorder