I would say transphobia absolutely is radical. Of course, by the definition in which it is extreme. Say, for instance, by these definitions:

Departing markedly from the usual or customary; extreme or drastic.

Relating to or advocating fundamental or revolutionary changes in current practices, conditions, or institutions.

The slang form of radical (being a synonym for excellent/wonderful) is merely a single definition, and when talking about politics (I hate to liken transphobia to a "political" issue, but it is heavily intertwined with politics now), its denotation is that of the quoted definitions above.