Both are around until roughly the first week in September
The Xenoblade Chronicles 3 icons will come in waves, and currently feature main protagonists Noah, Mio, Riku, and several side characters [heroes]. Wave 1 is available until August 4 at 5:59PM PT, and the entire line will cycle out on September 1 at 6PM PT.
As for Mario Kart Switch icons, you can find creations centered around booster course [season] pass waves 1 and 2 now, through September 5 at 6PM PT. Animal Crossing September birthday characters are part of the rollout too, and run through September 5 at 6PM PT.
As a reminder, if you want physical rewards, you can find our full Platinum Point guide here, which has been updated with details on weekly missions: bookmark it for when you need quick points for a reward. A handful of points is incredibly easy to get, as that’s basically just 15 minutes with Super Mario Run, or a quick sign-on to the 3DS or Wii U 3Shop. Also remember that any shipments are bundled together, so you can pay a flat rate for multiple items instead of paying it each time. Just know that items go out of stock on My Nintendo frequently, and some are never restocked.