Even if some queerfolk use cis in a snide way (which, ok, let's see it?), it's totally invalidated as a slur by the power imbalance. Cisfolk are not being hurt, losing jobs, or otherwise being discriminated on in the same way that real minorities are with their slurs and discrimination.
Saying "cis" is a slur is like saying "cracker" is a slur, but even more ridiculous.
Banh mi