It's entirely clear to me that any similarity was thoroughly accidental and that you should apologise to the cartoonist for going off the deep end.

Being oversensitive when you clearly hadn't taken the effort to understand the point of the cartoon (as is obvious from your letter) harms the cause by reducing the signal-to-noise of complaints about genuine racism. "Oh, all those racism complaints last time were ridiculous; let's ignore them." A better way to make your complaint would have been to take the effort to understand the point of the cartoon and then send a letter saying "you may not be aware of this, but your cartoon is uncomfortably close to this racist trope, and you should take more care to avoid this in future".