From the Super Mario Wiki
Sayonara is an Oriental city located somewhere on Mushroom World that was featured in two episodes of The Super Mario Bros. Super Show!, "Mario Meets Koop-zilla" and "Karate Koopa". In the former, the city was a modern metropolis filled with large buildings, though the latter portrayal displayed an older feudal town. Many Ninjis are shown to reside in the older-looking incarnation of the town.
One possible explanation for the severe difference in appearance is that in "Mario Meets Koop-zilla", most of the town is entirely destroyed by Mario and King Koopa. Therefore, it's not unreasonable to assume that the simpler design featured in "Karate Koopa" might be a result of fewer buildings existing, combined with the fact that there wasn't much time to rebuild since the group's last encounter with King Koopa. Another possibility is that the two episodes simply depicted separate areas of the city.
Karate Koopa's Attack
Some time after banishing Koop-zilla from Sayonara, Mario, Luigi, Princess Toadstool and Toad returned to the city to seek out Misaki, a martial arts expert whom they hoped would be able to defeat King Koopa and save the Mushroom Kingdom. However, the group was ambushed by Karate Koopa and his Ninji minions, and as a result, Princess Toadstool and Toad were captured.
Misaki soon found the group, and managed to at least rescue Mario and Luigi from the Ninjis. Misaki then proceeded to teach Mario and Luigi Plumb Fu, and after enduring several trials, they were able to save Princess Toadstool and Toad. Mario also managed to scare Karate Koopa into escaping through a Warp Zone, driving him away from Sayonara once and for all.