Hypocrisy means that what you are signalling is not reality. It doesn't harm you, directly; but it does, potentially, and in general, harm anyone who relies on your signalling.

Therefore, in a sufficiently large and inter-connected society, the society will be more successful if hypocrisy is given some significant negatives, like social ostracisation for known hypocrites (that also cuts down on the potential damage radius).

Therefore, societies which punish hypocrisy will, on average, be more successful than societies which do not.

So I don't think that the idea that hypocrisy is bad is arbitrary. It might not be obvious, but it's not arbitrary.