The babies would cry in church (or wherever) sooner or later regardless of what they were wearing.
The miserable people sitting in Costa are old enough to buy a proper jumper that isn't ironic if they like, nobody is forcing them.
Onesies are better on people who are completely potty trained and dry all night.
And I say this as the only member of my extended family not have inflicted antlers, hats or festive jumpers on their dog*.
I do object to some of the Christmas jumpers though. Ironically shite but naice is better than just plain, ugly shite that is not naice. I never realised I was a Christmas jumper snob before and I don't even have a Christmas jumper 
*nb - tiny dog has very thin fur and he feels the cold. He has a proper dog coat for cold weather that is designed only to stop him from freezing to death and in no way is meant to make him look cute although he does.
Said coat is apparently what mountain climbing dogs wear, and it has a picture of three mountains and a wolf on it. I believe it is the canine equivalent of a 'The North Face' anorak and I laugh at tiny dog every time he has to wear it. He is about four inches tall. He's never going to climb a mountain or be mistaken for a wolf, but he's also not going to freeze to death while doing his business in cold weather.