Light roasts are great! But there are just a lot of straight-up bad renditions of them, as a result of lack of adequate training on either roasting or brewing, to the consternation of many of those coffee snobs. Unfortunately, this just happens when shops follow trends.
Roasting well in general is already quite challenging and is a lot more than just arriving at a certain bean color or temperature. Vegetal flavors are very much a roasting mistake that's being passed off as an inherent characteristic of a light roast. Combine that with techniques better suited to brewing (or pulling shots of) darker roasted, and you have a recipe for a dull, astringent, sour cup.
That being said, a sour espresso shot is always possible regardless of dark the coffee is, so I'd argue it has a lot to do with a cafe owner's willingness to train themselves and their staff to work with lighter roasted coffee.
