That Shanna is doing the Head Alien voice to make fun of him is still one of my favorite things, and I totally believe she can pull it off.  It’s a bigger question why nobody else tries to mimic it.  Besides Dorothy, anyway.

So, about that “Dexter” reference. As David says, I wanted it there because I was heavily into that sort of winking mix of canon and fanon at the time, but it felt like it could also be a good variation on the general theme of Shanna vs. Head Alien, which is “pop culture references all over the place.” This is significant growth for Shanna, who would once have thrown a shrieking fit before admitting that she knew what a lightsaber was.
She references 1984 when she’s scared and then hits HA with at least two short-joke references: Calvin and Hobbes before this and The Wizard of Oz here. “Dexter” can be read as short joke #2 of 3: a reference to Dexter’s Laboratory and its eponymous diminutive genius whose reach usually exceeds his grasp. So it worked on meta and literal levels. (Plus, sure, it could’ve been “poindexter” too. I never object when readers find a totally workable other interpretation.)
As David mentions, the Fans‘ world has at least a few media properties that our world doesn’t have, most notably StarTec, a near-clone of Star Trek with a female transporter chief named “Dotty.” When you read the series’ first page, you meet Dotty before you meet any of the main characters, so her existence is as canon as it gets.
But the more involved my stories got, the harder it got to include those near-clones in a way that didn’t leave new readers with puzzled frowns. I don’t think a “Spaceman Spike” joke would’ve landed as well, for instance. So at this point, Fans maintained StarTec but otherwise did that sort of thing less and less. So, yeah, Shanna probably did know what Degrassi High was. Hell, she probably watched some, when she was trying to convince herself that was the sort of thing she liked.

But over time, I found that the “near-clone media properties” approach was causing me more problems than it solved, so I maintained the existence of StarTec but started phasing it out in general. For instance, because the point is that Shanna is growing more comfortable with who she is, she shouldn’t be mangling these pop-culture names any more. So it’s “Spaceman Spiff” and “yellow brick road,” not “Spaceman Spike” and “silver brick road” or what have you.