In Japan, King Bowser is called 大魔王クッパ, or Great Demon King Koopa. Koopa, or クッパ (kuppa), is named for the Japanese pronunciation of a Korean soup , 국밥 (gukbap).
In the West, Koopas were retroactively changed to be the name of the turtle species with Bowser as their king.
They didn’t make all the enemy names sound like Goombah, just one, Goomba. Koopas having a similar sounding name is coincidence.
The actual name of the Goomba species, however, is likely to be, as you say, a reference to Italians. In Japan, they are called クリボー (kuribo) in reference to their chestnut (栗, kuri) appearance — this is supported by the even more chestnut-like appearance of the subspecies Galoombas (クリボン, kuribon) from Super Mario World.
It’s not random at all, naming things after food is pretty common in Japanese media. See half the characters in the Dragon Ball series. It’s also attested in Nintendo’s own documents, alongside other candidates for names taken from Japanese names for Korean food dishes.
Yes, Mario is confirmed to be named after Segale. No, that doesn’t make your theory about Koopas being named to sound similar to Goomba any less ‘random’.
It would also fail to account for the fact that (a) Nintendo is first and foremost a Japanese company, with the American branch relegated to managing marketing, sales, and localization meaning that (B) クッパ as a name predates the English naming of Koopa.
If Nintendo of America had as much influence as you suggest, then the name Koopa would apply consistently to either the great demon king or the turtles, and the turtles wouldn’t be called ノコノコ (nokonoko, in reference to the sound they make when pushed into their shells and bounced around) in Japan.
In the West, Koopas were retroactively changed to be the name of the turtle species with Bowser as their king.
They didn’t make all the enemy names sound like Goombah, just one, Goomba. Koopas having a similar sounding name is coincidence.
The actual name of the Goomba species, however, is likely to be, as you say, a reference to Italians. In Japan, they are called クリボー (kuribo) in reference to their chestnut (栗, kuri) appearance — this is supported by the even more chestnut-like appearance of the subspecies Galoombas (クリボン, kuribon) from Super Mario World.