What happens if I use a 60W rated USB-C cable to charge my Macbook from the 85W power brick? Will they still be able to negotiate the correct charging amount of 60W and not overload the cable?
What happens if I use a 60W rated USB-C cable to charge my MacBook
  from the 85W power brick?


Nothing.  It will work fine.

A watt is a unit of work (power) and is calculated with the formula:

Watts = Volts * Amps 


A cable is rated for how much current (amps) it can handle - volts mean nothing.  For example, using the same cable and a 60W load on 22 AWG wire (22 gauge) with two different scenarios


3V at 10A would fry the cable
12V at 5A would be fine


I could put 50,000 volts so long as I didn't exceed the amperage, I'm ok.

This is why a wattage rating on a cable is incorrect.

Given that the Thunderbolt cable conforms to USB 3.1 and the power delivery spec for calls for up to 20V at 5A or 100W, your cable will handle it.