The reason the classical guitar is tuned E A D G B E , known as standard tuning, is because it allows for the most possible chords to be fingered with as little movement as necessary, is more efficient, and each chord can be fingered in about 3 to 4 different ways in different positions on the neck. Alternative tunings actually limit what can be chorded compared to standard tuning, often biasing songs towards whatever key the guitar's open strum is.
No matter how the guitar is tuned, it would not make composition more difficult, and it could be argued, because of the limiting nature of alternate tunings, that it is easier to compose in alternate tunings.
But this is beside the point. I'm not even sure Mitchell often uses alternate tunings, but if she does, it is likely one of two or both drop D or open G, which are used for the lower bottom note to give more bass, but also to make songs in those keys much easier to play.
What I suspect is that Mitchell uses standard tuning most often, and if she needs to transpose to a different key, she'll use a capo. I also suspect she composes, like nearly all folk music, using I-IV and I-IV-V progressions and in the keys of E, F, C, Am, G, or D, if the capo was actually at the nut (meaning, she might finger a song in D, but because the capo is on the 3rd fret, technically the song is in the key of F).
I haven't listened to enough of Mitchell's catalog to say unequivocally, but I suspect the music itself is not necessarily what is original or so compelling about her work, but instead her poetries and rhythms are what makes her stand out among folk artists.
And just because we like something doesn't mean we need to invent reasons why something is the best. That is more or less tribalism and kind of ridiculous.