I think the confusion comes from the distinction between culture and counterculture. The irony (or lack thereof) lies with the observers expectations.

If someone's expectation is that a green Mohawk means someone is throwing off the cultural legacy and expectations handed down to them, then yeah, a toddler with a Mohawk is pretty ironic. If someone just thinks is a haircut that looks cool, obviously not.

If someone thinks or remembers when Mohawk had that meaning, they might read into the fact of it being changing to a simple mainstream aesthetic cosmetic option.