I would think that anti-matter is radioactive (Spock died from radiation exposure while working on the intermix chamber), which is what they arm photon torpedoes with.
No, it isn't, not by itself. Antihydrogen or antideuterium is no more radioactive than normal hydrogen or deuterium -- the only difference between the two is that they have opposite electric charges. Trek's various references to "antimatter radiation," or the Malon's "antimatter waste," are pure technobabble. You get radiation when antimatter annihilated with matter and was converted to gamma rays, pions, and neutrinos, but that's not the same as radioactivity, which means the decay of unstable atomic nuclei. The only isotope of hydrogen or antihydrogen that's radioactive is tritium/antitritium, and that's not used in warp engines.
Basically the whole idea of TWOK's "radiation chamber" was bogus, just some random piece they stuck into the set as a plot device for killing Spock. It makes no engineering sense in its placement (off to the side, unconnected to the reactor shaft) and there's no explanation for the source of all the radiation. Since the main engines aren't online at the time, I'd assume there's no annihilation reaction taking place, so there'd be no gamma/pions/etc. from that reaction.
As for photon torpedoes,
in theory, they should be powerful enough to count as WMDs, since by all rights the yield of an antimatter warhead should be substantially greater than the yield of a nuclear warhead. (A single gram of antimatter reacting with an equal amount of matter would produce a blast the size of the Hiroshima and Nagasaki bombs
combined.) In practice, however, photon and quantum torpedoes are often treated by writers and FX artists as little more than cannonballs or artillery shells.
And that's understandable from a dramatic standpoint. I mean, let's face it, realistically, if you have large quantities of antimatter, you don't need any more exotic kinds of WMDs. Hell, if you have
impulse drive, that alone gives you the potential to destroy an entire planet, just by sticking an impulse engine on a small asteroid and accelerating it to a high sublight velocity. So really, in a universe with the kind of tech that ST has, there should be no need for stories about new forms of weapon or destructive force that needs to be contained, like polaric ion power or thalaron radiation or other bits of gibberish. They already use technology on a planet-cleansing level on a daily basis. But sometimes they need to tell a story about a powerful, scary new weapon, so the sheer devastating potential of their everyday technology tends to get glossed over.
So the question about whether photon torpedoes and thalaron weapons both count as WMDs is kind of impossible to answer, since it's a clash of two incompatible assumptions. In a universe that realistically portrayed photorps as WMDs, there'd be no need to postulate a BS particle-of-the-week like thalarons in the first place. Conversely, a universe that does have a storytelling need for "ultimate weapons" like thalarons is one that's choosing to gloss over the destructive power of antimatter weapons and treat them as if they were conventional weaponry.