Yes, Ai-chan was a very wild 'cosplayer' back in the day. Hohoho.
It's very hard to actually target the leather straps on a helmet, though. Generally, if you are precise enough to target the leather straps, you'd be precise enough to stab or cut the places not covered by the helmet, such as the nose, the armpit, the inner part of the elbow, the joints on the belly or the space on the neck between the helmet and the chestplate. There are natural weakness in the armour that cannot be armoured as it would hamper mobility. If the person can cut such a tiny point as a leather strap, they can naturally target these natural points of weakness as well.
There has never been any accounts of any medieval or antiquity battles where a fighter specifically targets the straps of the helmet. It's much easier to stab the opponent in the face, up from under the chin or through the neck if you have the opportunity to get to that point.
This is an example of a Galea, a type of helmet that most people associate with the Roman legionnaire. Though this was only common after Marian Reforms, where the state sponsored military equipments instead of the citizens supplying their own. See how close the strap is to the face, to the point that you can't even see it. At this point, you'd be better off stabbing the upper face or from under the chin.
The greek helmets such as the pilos and corinthian types also have straps, though you'd be hard pressed to find it on modern representations. But those helmets had holes to attach straps, which were tied under the chin. If you were to look at them, you would not find the straps, as they would be tight against the skin and as such, cutting the straps would be the same as cutting the skin.
