For myself, your initial question is too general to give a universal answer. What a person thinks they (or someone else) "deserves" is subjective because so much of that is wrapped in emotional assessments that can be tied to our personal history and experiences. That in turn can easily interlock with a belief or need to be validated for that feeling we have.

The hypothetical you offer as a general idea for "underserved validation" is not something I would deem as an honest expression. As a rule, I do not reward bad behavior regardless of its application in the moment.

If I observe someone validating a person as you describe, my initial thoughts are to not take that support as sincere, but rather a form of pandering to cater to something they want. Red flags always go up for me if I observe validation ill placed. Go on any given news channel and listen to the "experts" validate the words and actions of public figures most would never deem acceptable. That's an easy example but applicable here.