Plenty of the terms used in association with political parties are indeed fairly easily classified as "positive" or "negative": if anybody wants to make a case as to how "untrustworthy", "arrogant" or "useless" could be anything but unambiguously damaging to a party, I'd love to hear it.
But "socialist" - the leading descriptor of the NDP - falls among a few terms which aren't quite so easily pigeonholed. Indeed, even in the U.S. with a generally more conservative voting public, it tends to be a polarizing term which is nonetheless seen rather positively by a substantial chunk of the voting electorate (italics in original, bold added):
Of these terms, socialism is the more politically polarizing – the reaction is almost universally negative among conservatives, while generally positive among liberals.Now, it's true enough that other labels like "progressive" might speak positively to a far larger set of voters - and the use of "socialist" so far has mostly been a matter of the Cons trying to brand the NDP for their own supporters. But if the absolute worst label the Cons can attach to the NDP is one that's still seen positively by a third of respondents, that doesn't exactly hurt the NDP's chances of starting on at least even terms against a party whose own base is in the same range.