Idealism of self basically means that you only see the good in yourself - there is no bad - and that is not present in The Fall of the House of Usher. At the beginning of the work, there is not much about the narrator; and towards the end, he comments upon how he is afraid for no reason, how irrational he is being.
In fact, there is no idealism at all in Poe's story. For all the good things he says, both about himself and about Roderick Usher, there is also bad, often one right after the other. For example:
Surely, man had never before so terribly altered, in so brief a period, as had Roderick Usher! ... Yet the character of his face had been at all times remarkable.
In one sentence, he describes the awful changes that had taken over his old friend, the "terribly altered" state of Usher. However, two sentences later, he says that his friend still somehow looks as "remarkable" as he had (though not quite the same - it is remarkable in a different way). The rest of the paragraph (which is quite long) is a description of Usher's appearance, combining both good and bad aspects such as: "lips somewhat thin and very pallid, but of a surpassingly beautiful curve."