The audible effect might be more apparent if you bow as the weight will almost act like a mute. Maybe you can try and let us know? Is that the original guitar mount?
I know, that is why I first checked it with the bow. A large part of my playing consists of improvised arco playing, so arco sound is very important to me. The mic plus mount only weights in about 36 grams, and it does not influence the sound, either bowed or plucked. I always find that a mute does not so much influence the sound of pizz playing, but does have a profound influence on the feel of the instrument while playing pizz. Bowed, of course, is a totally different story. That is why I first used the bow in order to check the eventual damping effects of the mic.
Oh, and as I mentioned in a previous post, this is the violin mount.
Cheers,
Vincent