Well, both HN and the business press are often negative - and on the surface Microsoft was buying a failing business, so it's a reasonable reaction.
A case in favor of the acquisition might go something like this:
1. Microsoft was far, far behind in mobile, with little hope to catch up through internal efforts alone. "Winning" mobile had the promise to unlock a lot of new revenue across Microsoft's other business units, "losing" mobile threatened to restrict Microsoft to the stagnating desktop market.
2. The industry was already consolidating (Google buying Motorola), there was a looming patent war, Nokia was sitting on a ton of patents, and was one of the few (if not the only) good target left for acquisition.
3. CEO of Nokia was ex-Microsoft, so Microsoft likely had a good idea what they were getting. (Conversely, this could mean Nokia CEO knew exactly how to apply lipstick to a pig to get it sold).
In short, limited time window to act, potential for dramatic growth for Microsoft if it works out, relatively small downside if it doesn't (IIRC the cost was something like 10% of Microsoft's cash reserves at the time). I'd take that bet to be honest.
All of this is speculation of course (I have no inside knowledge), and it was clearly not worth it in the end, but I can understand why a decision like this might have been made.