Aside from the false stereotype of the overweight, goofy looking, flannel shirt wearing, gun owner there is a fair amount of truth in this.
Well-regulated as defined by the U.S. Supreme Court in the Heller decision is close to what the cartoonist says (Heller pages 23 and 24) :
… the adjective “well-regulated” implies nothing more than the imposition of proper discipline and training. See Johnson 1619 (“Regulate”: “To adjust by rule or method”); Rawle 121–122; cf. Va. Declaration of Rights §13 (1776), in 7 Thorpe 3812, 3814 (referring to “a wellregulated militia, composed of the body of the people, trained to arms”).
Of course the cartoonist intended to portray the NRA as ignorant and ridiculous but instead demonstrated their own ignorance and prejudice. You should not be surprised. It is “par for the course”.