If Harris had just said she smoked marijuana in college you could consider this a non-issue.
Instead she invented the story about the rappers as well and described the experience as "joy", which means she embellished it to sound more cool, and shows her to be a hypocrite later in regards to marijuana. So no it's not a non-issue. If a Republican candidate in 2016 claimed to listen to some "cool" music in college that wasn't out yet when they were in college they'd be mercilessly mocked for it, and rightfully so.
Or she just mixed up two memories that happened 25-30 years ago? Big deal.