In most cases, yes — a Nintendo Switch charger and a phone both rely on standardized negotiation protocols, so plugging one into the other won’t damage either device. The practical question is speed, not safety.
USB-C and USB-PD were designed specifically so mismatched combinations fail safely: the device only draws what it’s built to handle, and the charger only supplies what the device requests. The exception is older proprietary connectors (like original Lightning-to-USB-A or barrel-connector laptop chargers), where compatibility depends on the adapter tip or cable matching exactly.
Expect a phone to charge — possibly slower than its dedicated charger, but not unsafely. Check the wattage gap only if charging speed matters for your use case.