I've only know it as mild and affectionate silly - grew up midland live in south and couple of northern cities and s wales.
https://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1453_uptodate3/page16.shtml#:~:text=Very%20gentle%20word.,have%20done%20better%20than%20that'.
Numpty. N-U-M-P-T-Y. Numpty. Gentle word. Very gentle word. It feels to me like 'silly billy' or 'you novice!' It means somebody who displays a lack of knowledge or a lack of awareness. I've often heard it used as an endearment - 'oh, you great numpty', meaning, you know, 'you should have done better than that'. 'Silly billy' is the best gloss for it, I think.
But in some parts of the country and in some people's usage it's a bit stronger and you can hear the difference in the tone of voice. I mean, I've heard somebody say about a driver of a car 'that numpty's talking on his mobile phone while he was driving' and that's a stronger usage, it means more, you know, 'that idiot', 'that prat', 'that numskull' - he's so stupid that even the simplest things are beyond him, that's the kind of nuance there.
So it has a wide range of usage from quite gentle to quite strong. And then it's developed, of course, as time goes by. I've heard the plural of it - numpties, especially for politicians, 'politicians are numpties'. The number of times I've heard that! And also a noun, an abstract noun, numptiness - I don't think that's used very much but I have heard it once or twice - 'the numptiness of our politicians'.
And in 2007, it was Scotland's favourite word. I CAN, the charity, did a survey of the favourite words of different parts of the country and Scotland voted for numpty!
https://www.theguardian.com/uk/2007/apr/04/britishidentity.features11
Derived from "numps", an obsolete word for a stupid person, rather than the more obvious numbnuts or numbskull, the term implies general idiocy.