It is most likely not. Cosmetic procedures, give you a certain aesthetic. Gender affirming surgeries on the other hand, are reconstructive in nature and the parts they create have function. A F-T-M, trans person, getting a new dick, is able to achieve erection - either organically or through a penile implant (And penile implants are primarily used for men, who have such severe ED, that neither the pill, nor something like the 3 shot works. So it is not just a trans thing).
If your argument is that because it is plastic surgeons performing such procedures, it is cosmetic, then why are reconstructive surgeries done for cancer, covered as medical necessities? For example, any surgery done on the face, for nasal or paranasal cancers, involves extensive facial reconstruction (because they basically remove your nose, your hard palette, etc, so extensive reconstruction to restore function is required). So not all plastic surgeries are cosmetic in nature.
The other question is if gender dysphoria is a medical condition. Certainly mental health related issues are medical conditions, and gender dysphoria could be one of them. If so, then is gender affirming surgery an essential medical treatment? I dont have answers for these, but it is for experts to comment on, and if it is indeed a medical condition, for which the surgery is appropriate treatment, then it should most definitely be covered regardless of what it costs the tax payer.