There's a kind of playful teasing that feel really welcoming and inclusive in person. At my local card shop I might crack jokes at a friend's misplay and they might make fun of some ridiculous pet card I'm running. This serves as a kind of play, a way of pushing your friends to be better while letting them know it's no big deal. It can work to break down prestige-hierarchies, so Andy doesn't get a big head because he published an article on SCG, it flattens the hierarchy and keeps people equal, it comes from a place a love, it's a way of saying "I see you as part of a community that we're building together."
There are two very common ways this can break down.
One is that people have trouble reading the room. When a new player shows up and they feel like an outsider, that same joke might make them feel less included, being laughed-at instead of laughed-with. When a player identifies very deeply with a particular card or archetype, or identifies very deeply with their results, a joke at the expense of a card or play mistake can be felt as a personal attack. The recipient isn't wrong for "not getting the joke." Rather, developing a shared sense of humor is a protracted, interactive process that involves an initial stance of welcoming/inclusivity, and all parties gently pushing on the edges of the relationship until boundaries and norms are established. This process requires a great deal of empathy, to notice which actions hurt rather than harm people.
Of course, empathy is very difficult online. Even giving your best effort, asynchronous text without physical cues can be interpreted in so many ways. On a forum there's pressure to gather your thoughts into a cohesive narrative, to make your /ideas/ more structured and understandable, which means consciously stripping away a lot of the cues and context that would be used to facilitate mutual understanding of someone's emotional state. As methods of communication get less formal, more realtime, you get more and more of this feedback. (think about what it even /means/ for communication to be informal, those "um"s and "ahh"s are loaded with metadata about your relationship with the words you're saying). From your best friends playtesting, to your local card shop, to a live-chat discord server, to a local informal facebook/reddit group, to a global forum, it gets harder and harder to recognize when what you say isn't landing.
It gets harder to notice when you're being shitty.
And think about your close group of playtest buddies. I bet one of them tends to go a bit too far with their humor, and you all just kind of roll your eyes and think "oh that's just Andy, he means well." Empathy isn't something everyone's good at. Not even in the best conditions. An asynchronous global online forum might be literally the worst conditions for empathy most of us will ever deal with (It's a bigger challenge in completely one-sided communication like professional writing). That means if you place any value on not being a shitty person, you have to be more gentle, more careful, in a forum like this, when you're dealing with people you don't know from other, more empathy-friendly environments.
The second way it breaks down is a second-order consequence of the first. Maybe I crack a friendly joke about your deck, but my lack of tact makes the joke hurt, and my lack of empathy means I don't notice your reaction. If Andy goes on being a well-liked member of the community and you still feel excluded, you're going to learn a different lesson. You're going to think that Andy is well-liked despite, or because-of, the fact that he makes fun of people. Maybe you're the target of the joke, or maybe you just see observe and misinterpret the dynamic happening between two other people. But you learn something else. Now humor isn't seen as a tool to flatten prestige hierarchies, but a strategy to improve your place within them, by proving how clever you are. Maybe you start to see humor as revealing a dominance hierarchy - A can make fun of B, but B can't make fun of A. In this case cruelty is is a virtue, the more cruel someone can be without retribution, the firmer their position in the dominance hierarchy must be. This may not be a misinterpretation because there are absolutely communities that operate on these rules, and someone who has been stuck in these communities might adopt this mindset as their default behavior as a completely rational strategy.
I'm strongly against this kind of community norm in person, but online it's completely broken. When people get a whiff of a dominance hierarchy they react by trying to climb on top, or by leaving. In an online hobby community the cost of leaving is very low, and it creates a vicious/virtuous cycle. People who don't like playing that game leave, constricting the community. Now people in the middle of the dominance hierarchy have fewer people beneath them and have to spend more of their time on maneuvering than community-building. That means the fighting gets more vicious, the hierarchy becomes more obvious, and the value provided by the community constricts, which makes the community less valuable for people outside of the dominance game, who migrate somewhere else, constricting and amplifying things further. People motivated by altruism or prestige hierarchies have no one to help or show off to. The whole process spirals and spirals until novices are afraid to ask questions and experts don't post ideas because ideas just get attacked.
This attractor-state isn't inevitable, but an online forum is more susceptible to it than other sorts of communities. So I, as the admin, and you, as members of the community, need to be a little more careful (with humor especially), and keep in mind that play is supposed to unify us, not stratify us.
If you're making fun of somebody online and you don't know how they're going to emotionally respond, try and dial it back until you really get a better sense of them. Yeah it can feel dry and sterile sometimes, but you can get to that playful relationship, you just have to build up trust and empathy and earn it first.
But this is me being charitable, and assuming that your actual goal is to have a good time with the person you're joking with. Consider that trolling is, by definition, an attempt to get a negative reaction out of someone.
If you're writing a post and you think "hey, is this trolling?". Just don't fucking post it.
If you're reading this and you're thinking "but trolling is funny!". Just fucking leave.