The colloquial phrase nice but dim designates a person regarded as good-natured but also not ‘bright’ intellectually.
What is interesting about this phrase is that dim, contrasted with nice, has the original signification of the latter adjective, since nice originally meant foolish, ignorant.
Via Anglo-Norman forms such as nice and nise and Old-French forms such as nice and niche, the English adjective nice is from classical Latin nescĭus, meaning ignorant, not knowing, from the verb nescīre, meaning to be ignorant, not to know, composed of ne-, not, and the verb scīre, to know.
These are the earliest occurrences of the phrase nice but dim that I have found:
1-: From the television programmes, published in The Guardian (London and Manchester, England) of Friday 7th August 1981:
THE ROSE MEDALLION. Starting a three-part dramatisation by John Foster of James Grant’s best-selling thriller, with Donald Sumpter as Home Counties private eye Harry, Dave Prowse as his nice but dim cousin who works on a building site, and whose discovery of a skeleton is the start of a mystery trail, Anna Nicholas is the American girl Selena, who may know something.
2-: From the review of The Border 1, published in The Sydney Morning Herald (Sydney, New South Wales) of Sunday 2nd May 1982:
Nicholson is backed up by a range of excellent performances — Valerie Perrine as his nice but dim wife Marcy, Harvey Keitel as his manipulative friend, Warren Oates as the head of the border patrol.
1 The Border (1982) is a U.S. drama film directed by the British filmmaker Tony Richardson (Cecil Antonio Richardson – 1928-1991), starring the U.S. actors Jack Nicholson (born 1937), Valerie Perrine (born 1943), Harvey Keitel (born 1939) and Warren Oates (1928-1982).
3-: From the television programmes, published in The Observer (London, England) of Sunday 30th October 1983:
THE LADY EVE (film, 1941) 2: Perfectly judged comedy centering on an hilarious cat-and-mouse game between a con man’s artful daughter (Barbara Stanwyck) and a rich brewer’s nice but dim son (Henry Fonda).
2 The Lady Eve (1941) is a U.S. comedy film written and directed by the U.S. playwright, screenwriter and film director Preston Sturges (Edmund Preston Biden – 1898-1959), starring the U.S. actors Barbara Stanwyck (Ruby Catherine Stevens – 1907-1990) and Henry Fonda (1905-1982).
4-: From the review by Russell Baker of The Triumph of Politics: Why the Reagan 3 Revolution Failed, by David Stockman 4—review published in The New York Times (New York City, N.Y.) of Wednesday 30th April 1986:
Having left Washington a chastened man, he [= Stockman] doesn’t hesitate to do some chastening of his own in telling what he thought of the people surrounding the President (not much) and even of the President himself (nice but dim).
3 The Republican statesman Ronald Wilson Reagan (1911-2004) was the 40th President of the USA from 1981 to 1989.
4 David Stockman (born 1946) was, from 1981 to 1985, the Director of the Office of Management and Budget under President Ronald Reagan.
In Britain and Ireland, the phrase was popularised by the name of Tim Nice-But-Dim, a good-natured, upper-class simpleton character in the BBC television series Harry Enfield’s Television Programme (1990-92) and Harry Enfield & Chums (1994-97).
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