Because Janis Joplin is a singer from the '60s while Groundhog Day is a film from the 90s, so knowing the former has nothing to do with knowing the latter, as they're part of different fields hinting at different passions (unlike, say, if she knew of Janis Joplin but not of Joan Baez; or didn't know about Groundhog Day but did know about What About Bob?). Tree clearly is not a film buff. Also, it was meant as a meta joke (possibly even a joke by Tree at Carter's expense).