Of course it is.
I have no problem with fat people. I don't have a problem with anyone. Until it causes a significant problem for me.
If you are so fat it impinges on others - including risking the safety of others because you can fit in the seat, yes we should be able to a) talk about this b) take action to ensure others are not affected.
Culturally this is a British issue. I noticed on a trip to France at Easter there were posters in one city on the tram network about obesity and it being a 'national health emergency'. The photos used of individuals were not that big. I thought the campaign was appropriate and thought about how it was too taboo for us to be doing similar in the UK, not least because a third of the population here are bigger.
We absolutely should be saying no this isn't ok, rather than panicking saying that they can't help it, we are being so mean and cruel. Yes we can help it. That's the point. If you aren't prepared to make significant life style changes which plenty of other people do take seriously, it shouldn't be others problem.
Airlines have to consider the weight of passengers purely because of the laws of physics and cost of fuel. Even if you are 6'6", there is still a reasonable weight for an individual. Anyone above that, should be charged more and should be allocated appropriate seating for the safety and comfort of all.
I do wish we weren't so squeamish about something which needs to be addressed. Just get the fuck over it and yes, take action.
If you knew you might not be able to go on holiday or you might not be able to travel for work, that's a hell of a motivated to actually address the problem.