In Stranger Things, the Upside Down itself has no confirmed in‑story creator yet; the show has not revealed who or what originally made that dimension.

In‑universe answer

Within the canon, Eleven did not create the Upside Down; she opened a gate to a pre‑existing dimension when she contacted the Demogorgon at Hawkins Lab. Later seasons show that the place already existed and that other groups (like the Russians) could also access it, which reinforces that it predates Eleven.

The series has also not confirmed that Vecna/Henry, the Mind Flayer, or any other entity created the Upside Down from nothing; fans only know that powerful beings there can reshape or control parts of it, not that they originated the dimension.

Behind‑the‑scenes context

Interviews with cast and creators indicate that the Upside Down should be understood as an alternate dimension that the characters discovered and then partially altered, but not one definitively “created” by a human character like Eleven. As of the latest public information, the original creation of the Upside Down remains an open mystery left for future or final‑season storytelling.