While I wouldn't call Amblin' great, it's certainly good. But these things fall into a different category...the filmmakers are young and emerging into the genius' they'll become. Amblin', TDDS, and Bedhead are lightyears better than most short films, which tend to be horrendous, without even the production value to save them.
TDDS has shitty production value because that's what Anderson was going for. He's dealing with stripped down and bare characters and his camera, stock and lighting convey that perfectly.
Amblin' is a bit pretentious and misguided, but the virtuoso technique on display by a then 21-year-old Spielberg is superb and it's obvious why Universal hired him after seeing it.