Finding poles by inspection
The capacitance tied to AC ground is C1 and the input impedance of M1
is extremely large. Does that mean that the 1/RC pole is extremely
low?

The input source impedance will define the resistance that is in-effect, in parallel with gate source capacitance. For a voltage source (such as \$V_{IN}\$ shown in the question), that is usually zero or very small for a practical source.