It is more of a personal quality - somebody unlikely to cause trouble or to be aggressive.
To refer to something not human we would tend to use 'hwn' or 'hon'
So for a park as a masculine noun we would say 'Hwn yw'r mwyaf heddychlon'
This is the most peaceful.
Does this have anything to do with the origins of the word Heddlu? Peacekeepers maybe?!
The common root word in Welsh here is hedd (peace), perhaps more usually seen as heddwch.
Heddlu is a compound of hedd and llu (a crowd, throng or force of people).