Greta does get called mean things not because of her beliefs, but there is something about her (personal) behavior that makes her unlikable to many people.

The "she is unlikable" argument is just working backwards. It is a mechanism to criticize climate activism without having to talk about the ways in which climate change is a crime against our children. There is no possible behavior that she could take that would not produce this "she is unlikable" response except to never speak in public whatsoever.

You really don't have to personally like someone for them to have useful information

The nazi party member Pascual Jordan contributed significantly to quantum physics but it's rarely mentioned because of that association. On the flip side, also the nazis ignored his suggestions for advanced weaponry, to their detriment I would imagine, because he valued jewish scientific contributions and so was considered unreliable politically. (https://en.m.wikipedia.org/wiki/Pascual_Jordan, discovered via AstroGeo podcast)

Also consider that you're on hacker news. Hacker ethics, or at least the version I've internalised, include judging people by what they say, not who is saying it (race, gender, and authority are commonly mentioned, but charismacy could also easily fall in that category)

Are you sure you are not just buying a narrative being pushed to you? Who benefits from Greta Thunberg the outspoken activist against climate change and Israel’s campaign in Gaza being labelled a narcissist?

Yeah, her behavior of being outspoken and blunt. And female, of course.