So, hurting one's child, flaying and cutting their body parts for literally no reason is not a mortal sin? Let's all pity parents too indoctrinated to realize they are doing harm to their kids, but whatever you do, don't them it is a mortal sin to do so.
There is more than one thing going on with this topic and it might help us to clearly identify them.
Circuмcising in order to observe the Jєωιѕн ritual (knowing that the Church forbids it) is a mortal sin, whether or not one believes it is necessary for salvation. It is an expression of the heresy of judaizing. It is objectively gravely wrong, even in cases in which the person does not know better.
Performing an unnecessary medical procedure that mutilates the body is gravely wrong, possibly a mortal sin under some circuмstances. This is related to the Church's teaching on self-harm and integrity of the body. The majority of circuмcisions in North America are probably in this category.
Necessary medical procedures that remove the foreskin are not sinful.
Things are further complicated in that many people are having unnecessary circuмcisions done because they were deceived into thinking they were necessary. In this case, the sin is probably more on the ones guilty of deception.
Other than the first of these cases (Jєωιѕн ritual) we should be cautious about saying it is a mortal sin because the others involve questions of motivation and knowledge that we are unlikely to know.