It's a description of a behaviour, not a moral value judgement. It can be morally bad or morally neutral, or occasionally even morally good.
Morally bad trolling is disrupting a conversation you weren't involved in by posting something controversial, usually off-topic, that you don't actually believe but you know will cause drama, and starting a flame war.
Morally neutral trolling is things like posting pictures of Gandalf with the caption "Do or do not; there is no try. --Dumbledore", or wearing T-shirts saying "Helvetica" in Comic Sans. Personally, I don't see the appeal of inviting lots of random people to "correct" you just so you can go "Nyaha, gotcha, it's deliberate!" For me, the annoyance of the "corrections" would massively outweigh any sense of superiority from the "gotcha". But I don't think there's anything morally wrong with other people engaging in this kind of trolling.
Morally good trolling can be Socratic-type questions or statements designed to get people to examine their own biases or logical inconsistencies. An example would be autism activists talking about "people with allism" so that they can get people to go "That's a weird phrasing! I don't think of myself as a 'person with allism'..." so that they can reply "Aha, well, exactly!" Whether this kind of trolling is actually morally good depends on whether the cause you're doing it for is morally good, which people may disagree on, but at least in principle it's possible for it to be morally good.
But people have changed the meaning of "trolling" in a way that I think is unhelpful. I think the drift started with the meaning of "posting controversial things you don't actually believe, just to get a reaction." Because of the typical-mind fallacy, a lot of people are incapable of understanding that someone could believe a controversial thing that they themselves don't believe, so they think people posting controversial things must be doing it for the sake of trolling for a reaction, rather than for the sake of honest discussion. Also, the understanding that trolling is off-topic or at least derailing has fallen away from the definition, so someone posting an honestly-held unpopular opinion on political issue X in a discussion on political issue X is seen as "trolling".
Finally, because of the similarity with the word "troll" as in "ugly monster living under a bridge", the shift has progressed further: people with unpopular opinions are no longer just "trolling", they are "trolls". It's gone from a description of a behaviour to a dehumanising term for a person - exactly the opposite of the way modern psychology recommends labelling the behaviour, not the person.
(I know the phrase "don't feed the trolls", with "trolls" as a noun, has a venerable history; but when that was originally used it clearly meant "don't engage with people randomly injecting controversy to start flame wars", and there was a clear distinction between the regulars of a forum or community, who might have a range of opinions but could have productive discussions on the topic of the community, and "trolls" who were drive-by randoms. I think there's a difference between the etymological progression "trolling for controversy" to the shorthand "(don't feed the) trolls" on one hand, and "trolling for controversy" -> "posting honest unpopular opinions for serious discussion" -> "having unpopular opinions" -> "subhuman bridge-dwellers" on the other - especially when the latter is used not by established members of a community to describe drive-by randoms, but to describe previously-accepted members of a community who later admit to unpopular opinions, or used in a comment section, without a "community" as such, from one drive-by random to another.)
And this is especially bad when people don't realise, or deliberately deny, that the word has shifted in meaning, and thus repurpose discourse that made sense in the original context of "trolling" (don't engage with them, be strict about banning them from the community ASAP) to the new sense of "trolls" as "people who disagree with me".