The character is reflecting how is friend died recently. He comments that he used to be a guy who always messed around and yet he just died without warning in a freak incident...
ふざけてばかりいる奴だったのに
これが冗談や何かじゃないなんて
then in the last line, what is the actual nuance? "this isn't a joke or something..."
But what does なんて here add?
This use of “〜なんて” behind a full sentence implies some kind of wonder or disbelief. It's not necessarily positive or negative. It could be translated as something like:
To think this isn't a joke or something.
Unbelievable, this actually isn't a joke or something.
That this actually isn't a joke or something.
This sense of 〜なんて despite coming behind an entire sentence here incidentally also functions as a nominalizer and can transform a sentence into a noun ready to be used as a subject or object, but without any further “〜が” or “〜を” behind it. One could have also said “これが冗談や何かじゃないなんて信じれられない。” to mean “I can't believe that this actually isn't a joke.”, note how the “that this actually isn't a joke” in English functions quite similarly and also works as a nominalizer. One can also use “〜など” instead which functions the same and is more formal.
“なんて” behind a noun is a bit different, and can also be replaced with “〜など” or “〜なんか” or “〜なんざ” and more so adds vagueness or dismisiveness, similar to “or anything” or “and stuff” in English and doesn't have the same sense of wonder or surprise to it. This nuance of “〜なんて” can also be put behind a verb not behind a sentence, which has a different nuance such as say “愛してなんていない！” here it implies the speaker is dismissive towards the concept or even takes offense at the suggestion, something like “I do not love him or anything of the sort!” whereas “愛していないなんて” would mean “To think I actually don't love him.”
Since “〜なんて” behind a sentence typically replaces “〜だ” it can actually be syntactically ambiguous whether it's the sense that comes behind a noun or behind an entire sentence. “なんて” or “なんと” “なんという” in front of a noun or adjective however again adds a sense of wonder as in say “なんて美しい景色” to mean something like “How very beautiful a sight.”. This usage sounds a bit dramatic I'd say.
