It’s true, it was plagiarism, though accidental, in the sense that when copying Michelle Obama’s sentiments, which speech writers often do that sort of thing and that doesn’t count as plagiarism, the speech writer left in her actual phrases, she says by mistake. She admitted to it and apologized see Behind Melania Trump’s Cribbed Lines, an Ex-Ballerina Who Loved Writing

And Melania Trump did rewrite a professionally written speech, but not by herself. No suggestion that the plagiarism was intentional, as this sort of thing can happen by accident easily, maybe the speech writer copied passages from Michelle Obama’s to use as a reference to stimulate her own ideas, which would not count as plagiarism, but then some parts of it in by mistake. But it shows they didn’t do basic tests; there are tools you can use, even free online webpages you can use to test your speeches for plagiarism, to make sure this never happens. This shows that they never checked her speech in this way after she got it rewritten. How Melania Trump’s Speech Veered Off Course and Caused an Uproar

See also We ran Melania Trump’s speech through a college plagiarism checker