Is it dangerous to use an ungrounded USB C charger to charge a laptop?
Not likely to be dangerous to humans, but to devices if connected together.
Yes, it happens because it is designed to be connected to ungrounded socket, and for electromagnetic compatibility reasons it contains common-mode capacitor between unisolated mains side and isolated low voltage output side which weakly couples mains to laptop.
And yes, this is quite normal for power supplies which use ungrounded inlet. Especially no-name cheap ones can be so terrible you can't touch a device with metal case.
It should not be dangerous, and as long the laptop is only connected to charger there is no problems. But there is a chance that while the charger is connected, connecting it to another device like a grounded monitor there can be high potential difference when the connectors touch, and the capacitor discharge current can be so high you see small sparks. The connector must be good quality and hot pluggable so the voltage difference or discharge current does not go via data pins as it can do permanent damage.
Basically, almost all manuals of devices with ungrounded 2-pole plugs instruct to disconnect mains plugs of devices being connected, so I would do the same here so nothing breaks.