I don't think someone being funny when they sling shit is really an antidote to being unprofessional, but in itself I'm okay with someone being an asshole...I suppose I kinda have to be...what I have a problem with is someone being petty. I have no idea if the Coffman/Kurtz war qualifies as pettiness, I'm not a reader of either of their works so I've never seen it and don't care, but on the general level if I see an author engaging in petty squabbling or complaining or whining or what have you, my opinion of them and what they do tends to suffer. I think that's a big part of why I try not to talk on my comic about things that aren't my comic or what influenced its creation. I've been in many many situations where I wanted to cry and rant and bang on people's heads or call somebody's comic out for the garbage that it is, but I don't. It's not because it wouldn't be funny, I got the funny down, but because it would be really amateur if I did.
I think another thing, kind of in line with both this and the original points, is when authors make excuses about why their comics are late, or even when they're poorly done. Note the difference between a reason and an excuse. Of course it happens that people miss deadlines, it happens to us all, and not everyone has the ability to have guest strips sitting in a drawer somewhere. I'm not sure anyone would call it unprofessional when these things happen and audiences are generally understanding when they do, but it drives me batty when I see someone who hasn't updated in months and come back with a comment like "I had school and my parents were like such n00bs. O wellz LOL!"
It's not O wellz, you illiterate dunce! At least have the respect for people reading your work and the patience they show in dealing with your slow ass to apologize. Reading our work is not a privilege, it's quite the opposite. We have a contract with our fans that we will entertain them, and they pay us back by giving a shit that we exist. When we lapse in our commitment to that contract, the least we can do is mea culpa, take ownership and work to make it not happen again.
I think it might all boil down to respect. Respect for your readers, which you show by delivering what you promise and sending them a little fan service here and there and not being a stuck-up shithead when you meet them at cons and so on, and respect for your peers, which includes not spouting off petty snipes at them and showing support for their accomplishments. The more respect you have for your audience and your craft, the higher your level of professionalism is going to be.