This was a recent topic in this or the other forum I keep an eye on. Yes, the USB in the Bolt (and most cars) is very weak. My iPhone SE about maintains its charge level while running Google maps but doesn't really gain any charge. I believe when I tested it that it only showed about .94 amps which isn't exactly the normal 2.1 amps needed for decent charging of a phone or tablet. It's what GM put in and probably can't change it.
2017 Bolt EV Premier