I am a retired software engineer from a well-known company in U.S. Allow me to answer this question based on my personal experience and observation.

If you decide to find a job in industry, please don't limit yourself to only those companies which you believe would hire math PhD's. You never know. You should send your resume to the companies who offer jobs you are interested in.

However, you need to be aware of the expectations from your boss and co-workers after you start your job. Most people outside the math world have misconceptions about math. For example, they think a math PhD can do calculation very fast. So, if your performance does not meet thier expectations, you'll be in trouble.

Now, to answer your question, does your dissertation topic matter? No, particularly if it's in a narrow area your employer has no use. Is your math PhD useful to them? Yes. because they can use the general math knowledge and problem solving skills you bring to them. Your challenge is how to go from academia to industry and from math proof to real life applications. Once you overcome them, everything would be fine.