I haven't owned these headphones before so I don't know how the mic should sound but I would imagine that the mic shouldn't have that much distortion (especially for a flagship headphone). Some people suggest:
- Mute and unmute the mic before using it (probably won't do anything but worth a shot)
- Make sure that you haven't enabled any voice modifiers in your settings
- Check to make sure you still get this issue with the 3.5mm adapter
- Maybe consider returning it? If you have only bought it recently, you should be able to get a refund/replacement by either contacting Logitech support or the place you bought it from.