It isn't a case where if you punish someone for X you also have to punish someone for Y, which is clearly much less bad than X. The law is allowed to recognise that this is worse than just calling a person fat, just as actual people would. Some things meet the threshold for an offence while others don't.
And in this case examples of when it has been or is supposed to be used include things like persistently shouting obscenities, persistently pestering people in queues, bring rowdy in the street at night, some instance of swearing at police and brandishing "stop homosexuality" or "Islam out of Britain" type signs. I don't particulary think this instance is out of step with a law that is supposed to be applied to those sort of less serious offences.
Especially given that whether a mesage was "gratuitous and calculated to insult" is part of the guidance, and this clearly was given he had to actually get it printed in advance and then specifically wore it to a football game where there was increased likelihood of provoking reaction.