So, Mankar said that Mundus actually was just Lorkhan's plane of Oblivion and that Lorkhan is a daedra. What do you guys think? Is there something to it?

From a certain point of view, you could say Mankar is correct, and it is one of the few ways of reconciling the two differing theories of Sheogorath's creation.

EDIT: what he said with his serious face.

Sort of. The plane of a Daedra is, more or less, that Daedra's collected essence and a manifestation of its nature. Some might make the argument that Mundus is a plane in that it contains the collected essential nature of Lorkhan, but the difference between it and the planes of the true Daedra is that it was formed by Convention. Lorkhan's not the only god who gave his power to the Nirn. He might be the most crucial, as he was the one whose idea, most myths agree, it was to Convene, but he remains one of at least Nine.

The difference between Aedra and Daedra is that the Aedra are those spirits who sacrificed their own lives in order to create the world. As such, they are called the 'ancestors,' the literal translation of the term Aedra. Daedra, conversely, did not take part in the creation of Nirn and so remained whole and alive. Their planes exist within themselves. The word Daedra, then, means 'not ancestors.'

Lorkhan is not a Daedra because his heart and soul were torn out and used to create a world outside himself. Nevertheless, one cannot entirely discount the claim of Mankar Camoran. Lorkhan remains the principle Aedra of the Padomaic myths and the Mundus can be said with some legitimacy to be his 'plane.'