Is Electret Mic a reliable component to measure Sound Intensity Level (Decibel)
It may be repeatable but not very accurate compared to B&amp;K sound pressure monitor in an anechoic chamber.

Electret mics usually have an open back for far field cancellation,,so they are designed for near field sounds. They are not acoustically flat and thus may generate a poor comparison with different frequency sources.

But for your purpose for linearity, it may be ok, if you are not too loud or too close.