No, the couple probably did not have a Mario-themed wedding (well, not as far as I know!), but rather the term refers to how both Hoshino and Aragaki have recently appeared in Nintendo commercials.
Here’s Hoshino in a Super Mario Bros. ad celebrating the franchise’s 35th anniversary.
His recent song “Create” was inspired by Mario, it seems.
Aragaki is currently the face of Animal Crossing in Japan, appearing in spots for New Horizons and plastered all over the official Japanese site.
Over the last few years, she’s also been selling Ring Fit Adventure and the Switch in general to the Japanese public.
In Japan, commercials are a good barometer of how popular a celebrity is: the more big, mainstream ads they appear in, the more popular and embraced by the general public they are. (Of course, Japan is not unique in this regard!)
The couple didn’t actually meet through Nintendo; back in 2016, they starred together in a live-action TV version of manga The Full-Time Wife Escapist.