I think that you (as many others) have a distorted perspective on expectations and goals of doing research and writing a thesis in a educational setting. The following represents my vision on the topic and definitely is not an absolute truth. However, I believe and hope that many people would agree with my point of view. In my mind, education and research in educational setting, including writing a thesis (dissertation) seeks to enable a future scholar to perform research and, perhaps, teach. That is, the goal is to immerse a person in an experience and teach them a framework for approaching problems and finding solutions to those problems (gaining specific knowledge is a valuable, but secondary goal). That experience and that framework are much more important than anything else one can learn in an academic environment. As the famous Chinese proverb says,
Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime.
Obviously, this wisdom should be understood in the academic context, but both essence and value of this idea remain as valid in academia as they do in other contexts. Moreover, from the perspective of mastering the scholarly framework, I think that it is much more important to ask the right questions (and, perhaps, ask any questions, as IMHO there no wrong or bad questions), than to obtain results or, even, good results. In my view, asking questions that lead to obtaining results is paramount to becoming a good scholar and represents an essential part of the above-mentioned scholarly framework. Speaking of importance to science of asking questions and, generally, being curious (versus focusing on results), another famous phrase of a famous scholar comes to mind:
I have no special talent. I am only passionately curious. – Albert Einstein
Considering all the arguments, mentioned above, and returning to your specific question, I would say that it is completely wrong to gauge quality of a thesis (or graduate's achievements, in general) by research results. Not only results are not an indicator of said quality or achievements, but also "bad results" are as valuable, if not more, as the "good" ones. For more information on the value of negative results (for studies with formulated hypotheses), please see my other answer. Good luck!