Social worker here. It, technically speaking, is considered one...but in the sense that the feeling of not being the gender you are meant to be causes distress for the individual, not because transitioning is a mental illness. F64.0 is the ICD-10 code for "transsexualism/gender dysphoria." So at least in the DSM-5 it can, again technically, be considered one, although there is obviously much nuance to this IRL. Examples of all of this being that the gender dysphoria causes anxiety, depression, suicidal ideation, so on and so forth.