For example, dumping mercury into a major river causes extremely diffuse harm, but I'd never describe it as a "victimless crime."
In the other direction, smoking weed in private harms nobody except the smoker. The harm is so focused it doesn't even touch anyone besides the offender. Yet this is almost the canonical example of "victimless crime."
It's not about diffuse harm, it's about whether anyone was harmed at all besides the accused.