Unfortunately, the picture you posted does not actually show the offset of the rear wheel at all.
However, it does clearly show that the rack and the rear fender/mudguard are not in alignment with each other, as the rack appears to be a few degrees offset.
Drawing a line through the centre of the frame, shows that the fender/mudguard is lined up reasonably well with the frame, but the rack is not.
It should be possible to realign the rack with a small amount of "gentle persuasion", but as the rack appears to be made from aluminium, it might be better to leave it as it is.
If GM followed my original suggestion, the rim on the 20" rear cast wheel will probably be ~2mm offset from the centre
(check out this post for further details) as this was proposed in order to allow the same rim to be used for front and rear wheels.
My aim was that the front wheel rims should then be perfectly centred, ensuring the correct self centring/castor action of the steering, whilst the slight offset of the rim on rear wheels would not noticeably affect the steering/handling of the bike.
Alan