Note though that in the case of the current that is the maximum. The actual current being used at any one time will be some other number <= the negotiated maximum. (Some other magic cat /sys/... command may get you the current current.)
Personally, I use a USB power meter.
I guess the general answer to “is it safe?” is: yes, provided that both ends correctly implement the USB-C PD specification and the phone end is configured correctly. (Out of the box the phone end will be configured correctly so as long as you haven’t been issuing magic I2C commands, that aspect should be OK.)