There is very little GA traffic flying over Mexico City compared to other cities since this kind of traffic can't fly into MEX, GA operates from airports in nearby cities like Toluca, Cuernavaca and Puebla. Anyway, there are helicopters flying all the time below arrival and approach paths. More than GA and helicopter traffic, what you have to look out for is departing traffic when you are arriving, and viceversa, since there are several departure and arrival routes "intersections".
And that's other thing about MEX. There are very few cases when your aircraft receives radar vectors. Almost always, you follow an arrival route that takes you all the way to the Outer Marker of the ILS approach in use. Mexico City is actually in a valley, so there are hills and mountains that restrict arrival and departure routes.
It is very unusual to monitor the ATIS and hear something different than "visibility 6 miles due to haze and smog". Smog is worse during morning hours, so vis. of less than 3 miles is very often from 6am-11am. But of course, we also have our clear days.
And then there's the classic Mateo 6 arrival. You get to the Mateo VOR (which is in the northern area of Mexico City) via an arrival transition or a direct routing, descend from 11000 to 9700ft, and fly on a 160° track. During 9.2 miles, you fly over the world's largest city from north to south. Watch for the helicopter traffic approach control told you about, which can't go above 8500ft. Then you cross the MEX VOR 248° radial, and start a left turn to intercept the final approach course to 05R of 052°. Your heading is 160°, so this is no easy turn. All this while descending to 8800ft before reaching the marker, 5 miles away from the runway. You can see the Mexicana Tower, the Cruz Azul Soccer Stadium and the World Trade Center to your left (if you're a passenger, not if you're sitting in the cockpit flying the plane). "Continue to 05R, altimeter 30.38, you are number 2 to land". The challenge here is to cross the marker at the correct altitude, and avoid the common localizer overshoot or undershoot. Even LNAV and VNAV modes can have trouble doing this. You are now over Plaza LOM, landing gear is already down. Final approach. Runway in sight? Not yet, expect visual contact in 3 miles. What about landing clearance from the Tower? No, they don't clear to land here until there's absolutely no kind of traffic operation on the runway before your touchdown. Landing flaps set, 2-mile final, 05R in sight. Autopilot disconnect (if you chose to have it engaged all the way to this point). "Cleared to land runway 05R, wind 080 at 5". 300ft above the airport's 7341ft elevation. Minimums. You cross the displaced threshold, and there's traffic on take off roll on 05L. 50, 40, 30, 20, 10... touchdown. Reverse thrust doesn't have the same effect here, on a high altitude airport, so it's harder to bring the airliner to taxi speed. "Exit to the left, hold short of 05L, traffic taking off".
That's a typical landing in Mexico City International. Hope I didn't bore anyone... The thing that has me confused though, is how I began talking about GA traffic, which is what was being discussed in the post, and I ended up describing the approach into MEX.
"... for there you have been and there you will long to return."