That’s weird - both the underlying C library and python-igraph are single-threaded so it should not use more than one CPU core at any given time. However, in your screenshot it seems that you are running multiple Python processes in parallel, and of course each such process may occupy one CPU full core if given enough work to do.
Can you share a short, simple, self-contained example in Python that uses igraph and and runs on multiple CPU cores on your machine?