There are countless money exchange booths in the Mexico City airport, but I can't find one that exchanges Guatemalan quetzales (specifically that sells them). Is there one somewhere?
This is of course a question which is probably answerable best by experience, but since I think something is always better than nothing, here's a list of all the currency exchanges (In Spanish) on Benito Juarez Airport.
There is also an interactive map on the airport website which shows a list of banks and currency exchanges. They are mostly nearby, so I believe with a little bit of hunting, you might be able to find some GTQ.
Several articles also mention that the airport is probably the best place to find currency exchange at decent prices all across the city. One such article which claims that you should be able to find GTQ at one of the exchanges I listed earlier.