Part of why it works, at least for me, is the same basic trick that It's Always Sunny routinely pulls off. For one thing, no one in 8-Bit Theater is the voice of reason. It is very possible, in a given strip, to have all four of the main cast be reasonable at one point and then be completely unreasonable moments later as one of their own ridiculous assumptions bubble to the surface. As a result, you never get the sense of one person having repeated and exhausted responses to problems; you get to simultaneously laugh at someone pointing out how ridiculous the situation is while also not feeling bad that they're frustrated, because hey, if Black Mage is upset, we know that Black Mage is a sociopathic monster who deserves to be miserable no matter what.
The other element is that their opponents are just as bad when they're given any kind of characterization. They're either doing awful things to non-people (that is, individuals who aren't characterized in any fashion) or to people who absolutely deserve what's happening to them. And frequently they're the victims of their own hubris in that process.
"So, you didn't get any spells?"
"In the sense that being stabbed gets you a blade, man, I got spells."