Top 3 possible explanations:
1) Those are aliens, and Mexican is code for an illegal worker who is paid very little in America.
2) The "N--" word Iron Man is saying is "No other. Their work ethic is remarkable, I should pay them at least minimum wage and stop discriminating."
3) Batman is saying "careful" because he's afraid of pinching Iron Man's finger between the skull of the two civilians who seem to pose no threat. At least their concussions will make them forget Iron Man's very inappropriate comment.
Honorable mention to: Iron Man is racist!, They are bashing regular people!, and them most controversial thing on this illustration, why are they together? Batman is DC and Iron Man Marvel?
Correction: My brother who is much smarter than me (doesn't mean all that much but in this case he actually is really smart) told me that that was not Batman but The Black Panther. Silly me.