mosfet switching voltage spikes
The spikes are caused by the input signal edges leaking into the output through the MOSFET's gate-to-drain capacitance.

You can add a gate resistor, or a BJT which would be cheaper (don't forget the base resistor).

Another solution for 5V to 3V3 conversion is to use 74LVC logic gates which take a 3V supply but accept 5V on their inputs.