It is all a bit confusing. The
wiki says she is a "draconian wayfarer," not a shapeshifter per se. However, she gets categorized as type "dragon, storm dragon" and
not draconian (as opposed to the rest of the
monster draconians including Tiamat)... And not shapeshifter.
But then the lore quote says "will revert to her true form," which might be taken to suggest storm dragon (seems probable? "flooding unfortunate temples") or even shapeshifter (just because that's the corpse we get) I suppose.
She does often die in dragon form (I assume often and not always, unless the game technically changes her e.g. the moment someone pops a nuke on her initial form?), but what you get is a shapeshifter corpse not a dragon corpse so I'm not sure that makes it any more clear.
It seems pretty open to interpretation, if one doesn't assume the game categories are authoritative.
The only thing that's certain to me is she is definitely a unique... And she often leaves a shapeshifter corpse.