I'm well aware of the fact that mountain dulcimer players traditionally call DAD (or any other 1-5-8 tuning) a Mixolydian tuning, but if you play a tune with the open melody string as the tonic and the seventh consistently played on the 6+ fret, you are playing in Ionian mode, not Mixo. It's just a happy coincidence that the drones are tuned to notes that work in either mode. I suppose a 6+ could be sneaked into a Mixo tune now and then but I'd suspect it was an Ionian wannabe if very many were used.

The dulcimer tunings used for the four modes mainly used in dulcimer playing were developed before the 6+ fret became a common addition. Adding extra frets has thrown the whole modality question into a wringer, but the truth is that it's the tune that determines the mode, not the tuning of the instrument.