I think everyone is much more grumpy and scared at the moment, which doesn't help.
But also, 'petty' is a subjective judgement. If two people disagree over whether covid is an emergency situation or not, they're then going to disagree over whether objecting to people breaking guidelines is petty or reasonable.
You've got people who believe that the covid situation is a serious emergency, so they see some of the rule-breaking as just as antisocial and dangerous as drink driving. So they are sincerely and genuinely outraged by the rule-breaking - they're not being petty and vindictive over a tiny thing, because they don't see it as a tiny thing.
But then you've also got people who (for whatever reason) don't think covid is that much of an emergency. They don't see the rule-breaking as anything like drink driving but as more like a school-uniform violation or not buying a TV licence, so of course they equally genuinely and sincerely believe that the people complaining about rule-breaking are just vindictive curtain twitchers.
And never the twain shall meet...
That doesn't mean there aren't petty complainers out there, or that there aren't wilfully selfish rule-breakers. You get people making petty complaints about silly things in normal times, and you get people who drink drive. They're all out there. But I think an awful lot of the disagreements about covid rules stem from people having different understandings of the pandemic, not people's characters.