Yeah, Uma is Uma Thurman. Did she go to the same high school as J or not? (Amherst-experts raise your voice…) I think the whole joke is mostly about J having a crush on Uma. First he says that nobody is smiling anymore since she left, and when Kim asks about him and Uma it’s "too painful" to talk about. Especially because J looks and sounds so tormented (if you took it seriously, you might think he is about to start crying), the whole scene is extremely funny.
"Ever since Uma left, things haven’t really been the same. Everyone has their head down, and no one’s smiling anymore. The other day, a guy lit himself on fire and no one even cared – because Uma wasn’t around anymore."
– J Mascis