Based on my research, it appears that the developers of MKWii (and/or Mii Contest/Check Mii Out Channel) never made these flags, but rather sourced from an unusual Japanese car company named Honda. (Example)

At the time of MKWii's release, the flag of Venezuela [052, if you're a geek on MKWii mod files] only had 7 stars.

Note that Venezuela controversially restored an additional 8th star on March 12, 2006, to fulfill the wish of national hero Simón Bolívar, representing the historical province of Guayana (according to Britannica). Nintendo programmers over in Japan we're not even aware about the flag change at the time, yet continue to use the flag that Honda used up until the release of Super Mario Maker.

The flag of Netherlands Antilles [038] (created by the C.I.A.) also appears prior to country's dissolution in 2010, of what is now Curaçao (meaning Aruba [011] takes the win for breaking up in '86 and got it's own flag which even appeared in MK8 Deluxe).

As time passed, these designs became somewhat harder to find, duplicate and recreate the flag colors of the aesthetic feel of MKWii online play in this current era. Also to point out, Nintendo only chose these flags of select countries that have WiiConnect24 (which shut down in 2013).

Unfortunately, since the sources of these color modified flags are poorly sourced outside of Japan, it is currently unknown who made these color modified flag textures. Whether it could be from a Japanese exclusive CD-ROM, or from a Japanese website that has since been updated or no longer exists. So unless someone makes a YouTube video about this, the only thing we know as Non-Japanese users is that the devs at Nintendo most likely used these flags directly from Honda's website, which you can find here (keep in mind that some were sadly not archived since you know that nobody wants to archive a car site full of boring info...)

...or, if you want to see all of the country flags that Nintendo could've made into this game, check this one out for yourself!

@DZTVE It's larger and more desaturated than the other flags.

According to TCRF, the layout file for the Top 10 list (common_w078_ghost_top10.brlyt) references this unused icon as a placeholder for the flags, but it can't be seen in-game as other flags show up instead.

Why is the other Usa flag unused?

Finally.