Can a phase margin become bigger than 180° and is this bad?
What really matters is what the gain and phase are for a given frequency, phase margin is how much phase is allowed at a given frequency. If the signal is very attenuated (below 0 or something like -80dB, then most of the time it doesn't matter what the phase is).

A better way to look at it would be not to worry about what phase margin is, but how much phase there is at a given frequency. You find the 0dB point, and then look at the phase, if it is lower than -180deg then you have a problem, it will make the system unstable. 

Either find the 0dB point or -180deg point then find out how much phase margin you have. 


Source: https://www.sciencedirect.com/topics/engineering/gain-margin