I'm finding it odd that the tag is applies to Mario images that don't even refer to the titular game such as post #2496484 and solo images of characters that didn't even debut in that game such as Rosetta or Wario. And then it came to me that because of this, the Super Mario Bros. tag almost completely overtakes the Mario (Series) tag. Should something be done?

Agree. The following is my opinion on it...

Most of the [Super Mario Bros.] wiki could and should be migrated to the currently non-existent [Mario (series)] wiki, and all of the {Super_Mario_Bros.} posts should be checked and potentially migrated to more appropriate game tags, especially since there are more than a dozen tags that already implicate the (Mario (Series)) tag. Any images that don't specifically imply any particular game should only be tagged {Mario_(series)}.

Fixed up the wikis. Now to do something about the tags.