When a liberal calls someone "dumb", you could just as easily hear "crazy", or "mentally ill", etm. This is their language of erasure.
And it's not a surprise that this language has been weaponized by the right wing, because they gleefully latch onto the liberals' most effective rhetorical tools and turn them back on them.
We're about to enter an era when laws are written specifically to deny the "mentally ill" constitutional rights. But watch who gets defined as "mentally ill".