It’s also a golden rule of life: don’t be an arsehole. It’s universal, but telling the truth does not make you an arsehole.
Carl reminds me of some computer programmers I've worked with. They have zero social skills, so come across as exceptionally rude, but can't understand why people react so badly. They would also say "But I'm just being honest".
For example, Carl says this about his good friend's beer: his beer is garbage.
That's an incredibly arsehole thing to say about your friend and his fledging brewery. A normal person would say something like this: he just launched his brewery so his beers aren't ready (yet!) for this festival, but I hope to see him and his beers here soon.
So Carl, the problem isn't that you're honest, the problem is that you're just unpleasant.