It's important to place some context around your question. It sounds like you're still in school and are considering your first job after school. Employers look at fresh grads slightly differently than experienced staff, and in general, school-related attributes become less and less important as you gain more years of working experience. As a hiring manager, if I'm looking at people with 5 or more years of working experience, I hardly even care to know where they went to school or what they studied, in most cases.
While employers are obviously interested in finding candidates who have knowledge and skills in a specific field, they are also looking for candidates who can commit and get things done. People who are able to understand a problem, research and plan, come up with a solution, and then execute that solution are highly valuable. When looking at experienced candidates, it's somewhat straightforward to determine a candidate's ability to execute by talking about their actual work experience. When interviewing fresh grads, this is a much harder question to answer. You can expect a grad will have the "book knowledge" from their classes, but how to do know if they can actually get things done?
To answer your question in that context, the mere fact that you carried out an undergrad dissertation and were able to complete it will be the most important factor. The subject of that effort will likely be secondary, except in very specific niche cases (for instance, if your dissertation was related to solving a specific problem that a specific employer was facing, obviously that one employer would be interested!)
So - while you do want to pay attention to being employable, that doesn't just hinge on picking the right topic from the employer's perspective. It's important to pick something you're interested in enough that you will have the motivation to carry it out. And make sure you're staying mindful of lessons learned along the way - not just the engineering knowledge you're gaining, but also things like:
- How do you resolve conflict?
- How do you keep track of, and deal with, issues or obstacles?
- What do you do when your project depends on some other person and that person isn't delivering?
- How do you handle pressure and deadlines?
Being able to speak about those things in an interview, and show that you've gained execution skills in addition to engineering skills, will put you in a good place in terms of being employable immediately after school.