I honestly have never watched Invader Zim (though, my three-year old and I watched Paul's clip. My son wanted to watch it over and over and over again). But, anyway, since Gir seems to be our mascot here, I did some research on the Invader Zim wiki. It appears he likes
bacon (so much so that he put it in some soap...though since soaps used to be made with lard, bacon in the
soap isn't too far-fetched), as well as
pizza, and the burritos, tacos, salty lemonade and
poop soda sold at
Krazy Taco.
The wikia says that Krazy Taco is based on Taco Bell, so I guess that rules out authentic Mexican tacos (though Gir might like those, too). Since Gir likes bacon, pork tacos would probably be enjoyed by him. Maybe even add some bacon bites to make them "crazy"!
Other fun things for your party might be some bacon soap.
This bacon soap looks affordable and fun, even if it doesn't have actual bacon in it. Having a bar in the bathroom might be a hit with the kids, though! I know you said you weren't good at cooking, but I did find a recipe for making bacon soap (
https://sustainablescientist.net/2013/03/04/making-bacon-soap/) if anyone else is interested in putting their bacon grease to use in honor of Gir Bot.
You could also get some soda or kombucha and put labels on it to say "Poop Cola." Or, wrap chocolate bars with labels to say Poop on them, like this:
This appears to be the templet for the above candy bar, though I'm sure just taking some green cardstock and writing on it would work, too.
Even more affordable, you could make some lemonade and add some salt to it, to make "salty lemonade." I went looking for recipes, just for fun, and found that Salty Lemonade is a traditional Vietnamese drink, also called chanh muoi. I found a recipe here:
http://wanderingchopsticks.blogspot.com/2007/06/chanh-muoi-vietnamese-salty-lemonade.html
I hope that helps, and I really hope your son's birthday party goes well!