The etymology of none-so-pretty is exactly what it seems:
Etymology: < none pron. + so adv. and conj. + pretty adj.
It means that there is nothing as pretty.
OED does have some historical references. The first references are three or four hundred years ago, but all the meanings survived until the late twentieth century and probably beyond.
a. A type of fabric used in dressmaking and furnishing, prob. a worsted or lightish stuff. Now hist. and rare.
1622–3 in L. M. Cooper Rec. Early Eng. Drama: Chester (1979) 346 Two ounces of Silke one yard and ahalf of None so prettir to make a new Auntient.
b. Chiefly N. Amer. A kind of decorative haberdashery tape (more fully none-so-pretty tape), often woven in two colours or patterned with simple motifs, and typically used to dress furnishings. Now hist. and rare.
c1700 in N. & Q. 10 88/1 Webb-Cane and Leather Hooping, Gartering of all Sorts, Nonesopretties, Pins and Needles, Inkle and Spinnel.
2. Any of several garden flowers, esp. (a) London pride, Saxifraga × urbium (now rare); (b) heartsease, Viola tricolor; (c) sweet william catchfly, Silene armeria. Cf. nancy-pretty n. Now chiefly N. Amer.
1731 P. Miller Gardeners Dict. I. at Geum, London Pride, or None so Pretty.