Maybe just trying to make a point.

Everyone can have a bad day. But, in a customer facing job you keep your bad day on the inside and be professional on the outside.

If the cabin crew member in question is having a bad day to the extent that they are incapable of keeping it on the inside then they are not fit for work that day - if this was the intent in the post then I agree with this.

However, sadly I think that in many cases grumpy customer service professionals are perfectly capable of keeping it on the inside, they just choose not too. In this case it's usually bad attitude = time for a new job.