Likewise, last year it was noted at certain points in time it was very clear that target campaigns run by or for Microsoft on Reddit and HN were working to down/up-vote specific posts and write replies in their favour. The results and details of this were posted at least two or perhaps thee times before fading away, they might be on the internet archive somewhere, if I get a chance I'll try and hunt them down.
Well, it definitely is one, but you seem to mean "a conspiracy theory in the sense often depicted in mainstream media: unsubstantiated paranoid ramblings, see also: tinfoil hat".
I mean, it is unsourced, but it's not an unheard of idea, and should be quite Google-able. Microsoft has engaged in astroturfing many times (last in 2014 AFAIK) so it's entirely possible.
NeoGaf discussion with a list of news reports on MS astroturfing:
(I'm not associated with NeoGaf, I found it on Google, I vaguely recall it having some GamerGate-related stigma attached to it so I added this disclaimer)
That's kinda the problem with it. The time and resources one would involve to prove the point is substantial. And for what result ? At best a few people will get offended and that's it. MS has corrupted governments, insulted competitions and spied on its customers and yet they have been forgiven. So even if we prove what's is now a feeling, that they are on a PR campaign, it will achieve nothing.
Now maybe people will also start looking at the current "gate is a fantastic person" trend from the PR perspective as well.