I feel strongly that it's the musicians, not the tunes that tend to set the level of ornamentation. I find that the entheuasiam, experience and energy of the musicians are the important factors in how 'plain' the tune is. ik now dance tunes best, but am sure it's true in a broader sense. Also the size and 'freindliness' of the audiance...
Case study: In a regular expert class (Scotish country dance), we have a few regular accordianists, from England and Scotland, playing for a small group of freinds. Tunes very rarely get played straight, regardless of nation origins, of the accordianist, or the tune.
Importance: People > circumstances > tunes > countries of origin