I feel that this character--and possibly only his name--is one of the biggest stumbling blocks the film had, in terms of limiting its publicity and confusing critics, though I know very little about the real history and am hoping others will pitch in. Here's what I think, based on un-intensive research such as Wikipedia on Cancer Boy (Controversy)
and the cited Ugo.com interview with Dave Foley
If Cancer Boy hadn't announced himself using those exact words, "I'm Cancer Boy", would people have gotten so upset and offended? If, for example, he'd said "I'm Jeff"? And Bruce McCulloch had toned the goofy voice and affect down, on top of it? Maybe. Maybe not. I find most illustrative Dave Foley's comments on a season 1 KITH skit, The Cause of Cancer
, saying this (please note any problems I have transcribing and I'll fix them):
"[The Cause of Cancer is a ] fantasy piece where we find out that the cause of cancer is Bruce, and that he's sorry and he won't do it any more. It's a happy fantasy where this is how we can get rid of this nightmare [cancer]. It's nothing to do with making fun of cancer."
It would follow that Cancer Boy is not a mockery of people who have cancer (who on earth would mock or laugh at cancer sufferers, and why
?). He's there to make a point, an illustration for why some people might take antidepressants for situational depressions, as his own parents do. They have two children who are very ill, one with terminal cancer, and one who has his heart outside of his body. The children's illnesses aren't the punchline. If there is one punchline/point (just one?) to that scene, I believe it's Marv the corporate lackey's use of Cancer Boy as a tool to manipulate Dr. Cooper to keep doing what Roritor corporation wants. They'll stoop to anything. Even using a dying child's thanks and admiration for Dr. Cooper to keep their antidepressant scientist from rocking the boat.