IMO he's a serious music historian. Many years ago he had a radio series about the history of US folk music and progressive politics, covering among other things the labor movement, WWII, cold war politics, racial equality and the anti-war movement. I thought it was quite impressive, demonstrating real understanding and insight.
And of course his one true hit was Sexuality. Given those currents in his career, hopping on the trans band-wagon must have been irresistible.
I think Bragg is instructive in that a large swath of the left is unable to evaluate causes on their own merit, the problem of the omnicause. The irony is that of anyone Bragg could have known better because it's a recurring theme on the movement-left (and there's no doubt he sees himself as a part of that heroic tradition), one which he's actually covered -- the nativism, the struggle to disavow Stalinism and later Castro, the political unreasonableness and radicalism of the New Left which caused its self-immolation in the late 60s. But for whatever reason (and maybe it is just straight forward misogyny) Bragg blind to the unworthiness of this particular cause.