All horses will appear to have heavier muscle on the underside of the their neck if lifting their head straight up... It is just part of how the muscles work.
That muscling doesn't always mean bracing against a rider (it's a common reason), some horses just tend to travel inverted on a daily basis (even unridden) and that can cause it too.
If the horse, in a relaxed pose, has heavier muscle under the neck than along the top, it is said to have upsidedown muscling. (Or inverted neck muscles), generally, if the neck also dips along the top and has heavy under muscle it is called a ewe neck.
A true ewe neck is usually a conformation fault, not a conditioning probelm as it relates to the inner structure of the neck. (There are shades of grey here as some people will say inverted neck muscles are a ewe neck, while others will maintain it is the actual shape of the neck structure). My understanding is a true, structural, ewe neck is not going to change no matter how you ride.
BTW... your mare looks just fine to me.