I know you're specifying amateur comedy, but it's not so clear what's up or down. A lot of people think Dave Chappelle punches down, but is he really?[1]
"Comedy is subjective, and it's offensive, it really is. And people are making these imaginary rules, like, 'Comedy can punch up, but it can't punch down.' No, comedy punches down too sometimes, sorry. You don't make up the rules for my business without consulting a few of us. This is just people deciding these random rules about my business, and they're not even the funny people at their office, probably." - Jerry Seinfeld
"Who decides what’s punching up and punching down? I have a routine about these comedians writing articles in the Guardian, trying to set the rules of comedy, insisting that we should never punch down. And I say sometimes you’ve got to punch down." - Ricky Gervais
Also, it's important to understand what the actual target of the joke is. Chris Rock sums it up as, "Talk about what they do, not what they are"[2]. I think that's a useful rule of thumb.
1. https://arcdigital.media/dave-chappelle-punches-up-55dfe906e...
2. https://youtu.be/OKY6BGcx37k?t=2533