Is it normal for a PhD candidate to be dissatisfied with their PhD thesis topic, if the field is not hot? I am a 2nd year PhD student in Germany. I am working on computational mechanics. It is on modeling the mechanical behavior of a composite coating. My background is metallurgical engineering. When I had started with the research, I was very interested in the field. It's a field which was a hot research area a decade back, but now it's not. 

As time went by, l have grown unsure of the implications of my research. I feel that my research is wasteful as I would never be able to publish in top journals like Nature and Science. As my research area does not belong to a hot field. I feel that I should have researched a bit more about the research area before coming for PhD. I think I made a poor decision.

All these concerns make me distracted and unenthusiastic about my work. I feel that even if I graduate, I won't be able get any good academic positions.

Are these concerns common among PhD students?
From your question it seems that the only reason you chose computational mechanics was because it was "hot", and you are now regretting your decision because the field is no longer "hot" and thus it's hard to publish in high-impact journals. This is the wrong approach, not just because it's impossible to predict what topic will be "hot" when you enter the job market. More importantly, as a general principle, you should choose a research topic that you are really passionate about. Since doing a PhD is a long, hard, tedious, and sometimes frustrating experience, you will be miserable if you do not actually love your research. What matters at this point to you is not whether such dissatisfaction is common; rather, you should carefully reconsider your decision to enter the field in the first place and reevaluate your values and priorities. Such introspection will help you decide your next step. Depending on which stage you are in your PhD career, switching to an entirely different field might be the best choice.