Board Thread:General Discussion/@comment-27894627-20160511021754/@comment-27701762-20160527231654

Yeah, that claim doesn't hold water, given not only issues of the relationships between humans and monsters, but the fact that "Suzy" is Clamgirl's neighbor's daughter. So if Mettaton was Suzy, then Suzy's parent would own what we refer to as Mettaton's house. And the neighboring house to that one is Napstablook's, which would require that Clamgirl is actually Napstablook or that Clamgirl lives in Napstablook's house, both of which are absurd on the face of it. Not to mention that Clamgirl specifically says that you do not and cannot meet Suzy.

The idea was probably built on a misconception about the coloration of the words. Yellow text is applied to a wide variety of dialogue lines, so the move from "there is a name in yellow text" to "that name applies to the Yellow Soul" is a vast logical leap that is not prima facie justified.