I just found the font to be similar to the one used in Software Toolworks' game. I recall SNES SMW having a bolder font, actually. I guess I confused it because I had in mind the font used for SMW's Message Blocks, not the thinner font used elsewhere. Also, the font in that game doesn't match perfectly the one in the screenshot, so I guess it's not that. It must really be the font from the original game.
The NES version of Mario's Missing uses a font that's kinda thinner than SMW's, while the SNES Mario's Missing has an ever thinner 4x8 one, so that's where the confusion came from. I should've been more specific, I guess.