AFAIU FreeBSD is not well supported in Conda-land. I think you can get Conda itself to work on FreeBSD but repos like conda-forge (used by default in CondaPkg) don’t routinely build for FreeBSD.
However if you create a venv yourself with matplotlib installed in it, you can get PythonCall to use it by setting the JULIA_PYTHONCALL_EXE environment variable.
Edit: I’d like to add optional “python-only” backends to CondaPkg which completely avoid Conda and instead only install Python packages into a venv. But that will be a way off.