Reading back through this thread it looks like an ongoing problem on a new car and you're in Canada. I'm another with zero mirror vibration problems on either of my cx5 5's (2016.5 touring / 2019 signature).
I saw where you were taking your car to another dealer to investigate the issue. What did they come up with? Same answer I suppose ... Nothing wrong / normal. Odd how if this is normal how come it's only the left mirror?
The 1st dealer said they compared to 2 other new cars on the lot and they were the same. I see you said you drove 4 other cx5's. What did you find? Were the 4 others the same as yours?
As far as getting Mazda to make you happy with your mirror complaint I don't have many suggestions other than taking it deeper ... Complaints, arbitration, whatever consumer boards Canada has, etc.
As for a real world solution to make the mirror physically stop vibrating how bout trying some type of small piece(s) of foam wedged between the edge of the mirror and the housing. I'm thinking of things like packing peanuts, or squishy foam from an electronics box, piece of foam mattress, piece of "pool noodle"
Also, @Hmade14 (and/or others with mirror vibration issues) could you have someone record a short video of the problem while your driving with the difference between just driving and music bass thumpin' ?
It's not the same type of vibration you get from turning up the base on your radio. For me, vibration of the driver side mirror occurs at speeds over 100 km/h regardless of the road conditions (i noticed this on newly paved highways as well). It basically creates a blurry vision on the mirror making it difficult to see the vehicles.
Interesting. 16k miles on my 2019 GT and I have never noticed this issue. It must be a defect in certain cars assemblies. They should just be swapping the mirror out which would likely solve the problem. I would keep complaining about it until they do something.