I can see you thinking it's in poor taste or poor judgement since it will make lefties heads spin, but how is it irresponsible? It's not like it's being advertised in Highlights or on Sesame Street. Kids can't buy guns, it's clearly marketed to moms and dads. I have serious doubts that any kid actually old enough to read would be somehow irresponsibly swayed into evil by a cartoon skull.
The whole "Marketing things to children even though A) they aren't in children's magazines or on TV, B) Children cannot buy them and C) it is a legal ( and constitutionally protected) product" is being exploited by anti-gunners and is a gross violation of 1st amendment rights in an attempt to violate 2nd amendment rights.
I completely understand that many people think it is just giving ammo to anti-gunners and is in poor judgement or even poor taste, but saying it is irresponsible is the same thing as saying that the anti-gunners are right and advertising a product like this, almost certainly in an adult, gun magazine, is going to somehow make little Johnny shoot up a school.