I think the insulting part of "cishet scum" is "scum." I definitely see "cis" and occasionally "cishet" as entirely denotative self-identifications in spaces where it's relevant and trans and non-heterosexual folks are at least more visibly represented than in the world at large, and it's neither positive nor negative, it's just what one is.

The phrase "male chauvinist" doesn't mean there's an intrinsic negative connotation in calling someone "male," nor does the phrase "nasty woman" mean it's intrinsically negative to call someone a "woman." (Of course, both terms might be used as insults in context / with tone / etc.)