As far as this pertains to therianthropy:
If someone experiences a sense of animality due to DID or some other origin that we might consider medical, mental or neurological and their experience hits most of the fundamental key notes of other therians, I think it's fair to consider it therianthropy. My reasoning for this is if you ask 100 different therians why they're therian, you're going to get 100 different answers. I don't think focusing on origin is useful in the slightest. Our ideas of where our therianthropy stems from are vast and far more random than similar. Our experiences, however, are measurably comparable and hold some fundamental similarities.
With that said - yeah, it's trendy to identify as plural in the otherkin, fictionkin, alterhuman communities. I've had this discussion before and I've come to concur with @elinox
that there likely are folks within the community who experience something like this but the ratio of genuine to disingenuous claimants is likely skewed heavily towards trenders, role-playing, trying to be special or making some kind of a performative, identity-politics statement.
I agree with Dust that this type of thing ought not be glorified but I also agree with many others that it's not something we should be attaching stigmatization to, either. The thing is we live in a world of polar extremes and it seems that no one can find it within their better reasoning to find a neutral, middle-ground on this stuff. Not stigmatizing or treating plurals poorly does not equate
to placing them on grand pedestals to the point where young, impressionable people start saying "I want to be diagnosed with DID when I grow up!".
There are many harmful trenders, imposters on and offline for a number of self-serving reasons. People will claim to be something they are not to feel special, in seeking social acceptance, for social security fraud, attention, sympathy, ect
I find the argument that "everyone is valid just because they say so" incredibly ridiculous and naive to the fact that deception exists and people do it if they can benefit from it in some way. I don't think accusing individuals of being deceptive on a whim, especially if we aren't part of the group someone is claiming to be, is a good practice. What we need to do instead is stigmatize the concept of playing make-believe for one's personal gain and talk about how it's harmful/hurtful. Which, to me, is essentially what the OP seems to be aimed at.