Specifically talking about Smash Bros. The Smash Bros editor by @RandomUser is able to edit saves but in a crude way as he says:
So could they see it and ban me?
Keep in mind that the editor allows you to acquire all move sets, headgears, and outfits, in a very crude way. It simply writes "FF" to the location of the save file that enables these things. I do not know if Nintendo can ban based on what is written on the save file or not, so keep that in mind, that it is very noticeable when viewing it in a hex editor.
So could they see it and ban me?