Green’s Dictionary of Slang

nice nelly n.

also nice nellie
[SE nice + Nelly, a generic term for a respectable woman]

(US) a respectable or fastidious person (not necessarily a woman), also used ironically.

[US]AS XI 226: Inquiries concerning the meaning of two terms have been received. They are Nice Nellie and Charley horse.
[US]Berrey & Van den Bark Amer. Thes. Sl. §434.2: Respectable girl, ... goody-goody ... nice Nellie.
H.B. Summers Radio Censorship 262: Even one Nice Nelly objects to something he has heard over the air, the Commission may threaten to revoke the license of the offending station.
[US]H. Rawson Dict. of Invective (1991) 252: A nervous Nelly is a person who is easily upset or scared; a nice Nelly is a prude.
‘Ground Zero Grudgement Day’ Grudge Match.com 🌐 Wilson Wilson vs Ned Flanders: [...] Flanders, being a nice-nelly, will be denied the stress release of a torrent of swearing during the bout.

In derivatives

nice-nellyism (n.) (also nice-nellie-ism)

(US) prudishness, excessive gentility, puritanical behaviour or attitudes.

J. Mabie ‘Beautiful, but Dummies’ in Weekly Mag. Section 27 Nov. 🌐 Mr. Gaba thought if there was a little more naturalness and a little less Nice nellyism, it would make the windows look – well more inviting.
[US]P. Wylie Generation of Vipers 168: A shrinking-violet attitude, a pants-wetting nice-Nellie-ism, behind which modern doctors hide.
T. Pyles Words and Ways of Amer. Eng. 146: But, even if we discount the story of the pianoforte with pantalettes, there is yet abundant evidence of the nice-Nellyism of American English.
A. Hatch Lodges of Mass. 85: That was the last stringent hour of the era of ‘good taste’ or ‘nice nellyism’.
B. Elwin Sherman ‘Sherman’s March’ Elwin’s Humor.com 29 Oct. 🌐 In defending its closing the heaven’s gate retiree payola, Sears said it has: ‘Maintained its right to modify employee benefits.’ At best, this is a nice-nellyism for: ‘Up your nose with some rubbery prose.’.
Holliday & Hassard Contested Bodies 102: This nice-nellyism smacks of sexual services, an irony not lost on a European.