Anyone who thinks showing a mockup to a customer and asking “whaddya think” is any kind of user research or will result in anything other than passive agreement and nitpicking over small details doesn’t understand user research. You are assuming that any kind of attempt to validate without building a feature leads to this lazy form of so-called research. That’s not true.
There are MANY ways to collect user feedback and data without writing and releasing code but also without using printed out mockups and lazily asking “whaddya think.” The job of user research is to set a research question and design experiments that seek to provide answers while controlling for variables as much as possible. There is a reason UX researchers get master’s degrees — because such degrees offer training in research methods way beyond “here’s a mockup, whaddya think?”