Does the Pico request 500mA from the USB bus when connected to a PC? And what does it do when it's only allowed to use 100mA?
Anyway, assuming your USB port can provide 500mA that's enough for the Pico (32mA) and the sensor (70mA +/-10mA), no need for an external power supply.
When using multiple power supplies you MUST connect the ground of all power supplies together. Otherwise there might be a measurable voltage difference between the different grounds, e.g. due to power supply design or due to electrostatic charge.
-> Remove all data cables before connecting grounds together, otherwise you might damage your chips.
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