I remember asking that same question, but I don't remember who told me and they didn't tell why it was haram, I remember it was because your not allowed to have like part of your head shaven and part not, like marine style (sides shaven and top not) is haram and I think the mohawk would be in the same category as marine style hair cut. That is all I know and not very liable source, best ask a sheikh. Also as long as it doesn't offend people and your not a muslim do it.