I have yet to see any gunsmiths listing either Amish or Mennonite as their religion (though maybe the infamous Martin Meili was Mennonite.... I'd have to look again and see). Generally you see Reformed, Lutheran, or Moravian.
I would actually expect to see some Mennonite gunsmiths perhaps, here and there, but I would not expect to see any Amish gunsmiths in the 18th/early 19th century. At the time, the Amish were the more austere, breakaway sect.
As far as being "good craftsmen".... well, some are, and some ain't. Just like with anyone else.
Oh, and among the Amish/Mennonites I know, they do tend to like pretty nice guns!