To me, un-American is a label that’s usually used to try to argue against something, and usually seems to be used in ways that I myself would consider un-American.
I think of the positive side of Americanism as being about resisting oppressive power structures, for the sake of life, liberty, the pursuit of happiness, freedom, independence, being allowed to live, think and speak as we please as long as it isn’t doing harm, freedom of religion (or non-religion), the equal and unalienable rights of people, and general good will.
I think of the negative side of Americanism as what I would call un-American, but I don’t usually use that term because it seems like a crude and divisive way to argue that is usually used by people I wish would all collect someplace I don’t live, and/or stop being that way. That is, people who want to assert/inflict some dumb-ass version of Christianity on everyone. Or “capitalism” fanatics. Or racists. Or warmonger “patriots”. Or anti-Islamic fools. Or homophobes. Or anti-intellectuals. Or anti-introverts. Etc.