I know a
Bronwen who has always been called
Bonnie (and by now she's married with kids). Her family did nns in a big way.
To me, though,
Bonnie is more an adjective than a nn. I named one of my Scottish terriers
Bonnie - she was a most beautiful puppy - but as we got to know her better,
Bonnie turned into Bonzo to suit her personality.
Bonnie, in British and especially Scottish English, means not only pretty but healthy-looking: a bonny baby would be plump and cheerful. A bonny young adult (always a girl by that time, though a bonny baby could be either) would be pretty and lively and, ahem, a sex object:
Robert Burns uses it to reference numerous young ladies, including bonny
Jean and bonny
Ann. And then there are the "banks and braes of bonny Doon" and "bonny Scotland" itself, as well as Bex's memory of "my bonny lies over the ocean".
The -ie and -y spellings are pretty interchangeable, I think: I'd expect
Bonnie and bonnie to be more Scottish than not, and Bonny/bonny could be from anywhere. As a name, I prefer
Bonnie, but that doesn't mean that I like it - I don't. Except on dogs!