Each of the other mayors' puzzles contains three mistakes.
In the Dunceville mayor's crossword, there are three cells where the across and down clues don't match: The I/M in 8 Across RIO and 5 Down EMU, the P/U in 12 Across TRIUMPH and 10 Down TIP, and the E/P in 18 Across NET and 13 down RIP.
In the Dullardsthorpe mayor's Quote Square, the leftover letters spell out the Steve Jobs quote "People don't know what they want until you show it to them," but among these letters there are three leftover leftover letters, B, O, G.
In the Idiotsburg mayor's Rows Garden, there are three cells where the row and bloom clues don't match: the T/G in STALEMATE/GENEVA, the R/G in SARAH/GASCON, and the F/L in FED/KHALED.
The letters involved in these cells aren't as important as the locations of the cells themselves. To solve this puzzle the correct way, print out all three postcards, two-sided, on sturdy, glossy paper. Complete each puzzle and then use a kitchen knife to stab through the cells that contain each mistake. This will puncture three letters on the front side of each postcard, and those letters spell the answer.
If you are lazy, simulate a knife stabbing through the postcards by overlaying each front on each (reversed) back:
The answer is PHNOM PENH.