Realizing a small signal inverter
The mosfets are not properly biased.

The models you are using in the schematic are power mosfet, they can withstand some 100V Vds, with only 3V across each device they never really turn on. Try running a frequency sweep, not only this circuit does not invert the signal, it even attenuates it.

You can fix it in two ways:


use small signal mosfets, something with a 600mV threshold should do
power your circuit with at the very minimum 12V, 20V is better. Change Vbias accordingly


Simulators are powerful but you need to use them with a grain of salt. And always, always, always read the datasheet. Really, just do it.