You should be aware that many graduate courses inflate grades - much worse than undergrad courses - and this is a well-known fact in academia. Grades don't matter for graduate students and they rarely flunk out (they have to maintain an average of around 3.0 depending on school, which is quite simple given the rampant inflation). So it shouldn't be difficult to do well if you put in the time and effort - you shouldn't ever sell yourself short just because you're an undergraduate.
In terms of how it looks, that would depend on who looks at it. As I said, it's known in academia that grad courses are grade-inflated. But you're also showing initiative by exploring material that is interesting to you at a greater depth. I think it would reflect positively on you if you did well in the course, which is almost easier to do than in undergraduate courses.
