Using a mandolin for solo song accompaniment seems to raise eyebrows if not comments, especially from guitarists. But I think it's perfectly acceptable. Indeed, I like the sound and at the very least I find it a refreshing change from the ubiquity of the guitar. In particular, it facilitates melodic interludes which can be much louder and more effective than on the guitar.
Here are some examples from YouTube of mandolin use in solo song performances which speak for themselves. Readers will appreciate that I don't control the continued availability of these videos.
Bill Monroe - 'The Wayfaring Stranger':
Maunus (Mandolin Cafe) - 'John Hardy':
Steve Earle - 'Copperhead Road':
