I think it's telling that a lot of people who are claiming Jax is bad trans representation are cis people. If you are one of these people, you most likely have good intentions and don't want trans people to be portrayed in bad light. But you're not helping. You're alienating many, MANY trans people who relate to the repression that Jax exhibits.
Jax is meant to be a cautionary tale against denying who one is and refusing support when it's needed. A lot of trans people never tell anyone that they're struggling until it's too late. A lot of trans people have taken this warning to heart and opened up to loved ones that they trust for help and support as a result of the finale.
Worried cis people, your white knighting might be done in good faith, but it's actually harmful in the long run. Instead of asserting your own belief that Jax is inherently bad trans rep, read into what the story is actually saying and share why Jax's ending was impactful. Do not contribute to shallow readings of stories. Encourage deeper thinking so people can actually understand what authors are trying to convey.