As you may know Harvard Extension is the adult community education / outreach branch of Harvard.
Given that it's a university institution, your participation and grades there will be very well understood by colleges, including Harvard College.
The course will look worse if you:
- choose a class that's easy,
- choose a professor that gives a lot of A's (common in grad courses ironically)
- or are clearly doing it for signaling (there are lots of hard classes at high school you're not taking).
The course will look better for you if you:
- choose a class that's known to be hard.
- develop a relationship with the teacher and potentially get a recommendation.
- have a genuine reason for taking it at Harvard Extension without it being "too obvious" that you're doing to for the college checkbox. For example if your local high school does not offer a course in topology but Harvard Extension does, and it's the natural choice for you.
I would also add that a local community college with hard courses would serve you equally well.