Is it normal for a PhD candidate to be dissatisfied with their PhD thesis topic, if the field is not hot?
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.