I'd say it was a decision that was made when King was high off his ass on coke.
Moderators are terrorists.

Why the Green Goblin specifically? Why not literally any other kind of supernatural monster? Why not a clown and say it was a reference to 'IT'?
And if you think I'm looking too hard, I think you're not looking hard enough.

So yeah, does anybody know WHY the truck is modelled after the Green Goblin?
Why not? It provides a prominent distinction of which truck is "the leader", and it looks cool. If you're looking for anything more than that you're looking too hard.

Well it did look really cool

I haven't gotten around to re-watching it, yet. I'll pay close attention, when I do.
"You may have come on no bicycle, but that does not say that you know everything."

Used by permission from Marvel near the end of the ending credits.
(Family Guy)There is no Peter, only Zool.

The choice of using the Green Goblin's face on the truck isn't what strikes me as odd. What puzzles me is how they got away with using a Marvel Comics licensed character. Marvel are known to be highly protective of their copyrighted material, and I never saw a 'Thanks to Marvel Comics' or 'The Green Goblin is the registered property of Marvel Comics,' or anything like that in the credits.
Did Marvel overlook it because the movie bombed in the theatres?
You may have come on no bicycle, but that does not say that you know everything.

Maybe it's an "evil" toy company

Seriously though, no idea. Even with its flaws I've been a huge fan of this film. Not sure why it was picked, except it looks really menacing.
Maybe the toy company that owned the truck had a product launch for that specific toy at the time.
Conquer your fear, and I promise you, you will conquer death.

True, but again, it's just such a strange choice. As the Nostalgia Critic brought up, it's supposed to be the mascot for a toy company, which makes no sense unless the idea was to terrify the kids buying the toys. We even see a small jack-in-the-box which looks identical to the Green Goblin mask on the front of the truck, and they're both equally creepy.
It's just so confusing.

It may just have to do with the mythology of goblins. They are creatures who are mischievous who want gold (or gasoline, lol). I haven't read
Trucks
ever or seen
Maximum Overdrive
in probably 20 years, but King has a penchant for using pop culture figures as symbolism in his stories. He may have used the Green Goblin (Marvel license and all) simply as a fan and wanting to put that image on film since it hadn't been up until then.
All good things must come to an end - Chaucer

That's what I'm wondering. I know for a fact it's the Green Goblin, there's even an anecdote on the Spider-Man (2002) DVD, pointing out that the Green Goblin's first live-action appearance wasn't even in an official Marvel product, it was in 'Maximum Overdrive'. So it's certainly the official character, but
why
?

Yea, I remember recognizing the likeness as a kid and always wondering. I just looked it up on a whim and it is in fact the Green Goblin complete with a license from Marvel! I wonder why King did that? He's know for pop culture icons as symbols in stories (like the Rita Hayworth poster in Shawshank covering the hole in the prison cell.)
All good things must come to an end - Chaucer

I always wondered that myself.
When theres no more room in Hollywood, remakes shall walk the Earth.