Yamato is traumatized. That's what she is. She's not the conventional trans person. She was imprisoned, beat and starved by her father, so she coped by imagining she's her idol Oden, because Oden stood up to Kaido and scarred him. He's a hero in her eyes and she wants to imitate him, like a young kid wants to imitate Spiderman or Batman, but more.
Actual Trans people seem to identify as their gender purely because they identify as it. They sincerely believe that they are their new gender (and they are, ofc, my point isn't to deny their validity). A woman transitions and now identifies as a man, he is a man. A man transitions and identifies as a woman, he is now a woman. Yamato isn't like that because she only adopted this Oden stuff purely as a coping method, it's not something she just started to identify as on her own.
This probably sounds very transphobic, but I'm not at all trying to be transphobic. I think all trans people are valid, and I will call ftm people males and mtf people females. But I just don't believe Yamato is truly Trans, she just wants to be Oden because it helps her cope with how bad Kaido treated her. If she really was, then she'd likely have a problem with Luffy calling her "Yamao" and not Oden, for example.
Though I could be wrong. She could very well be a trans character. And in that cause, great. OP needs a few important diverse characters here and there, it can't only be Ivankov Inazuma Bon and Morley.