I think there are two
I think there are two possible explanations for this Cybertruck bashing on fan sites: (1) maybe the fans object to potential changes to the vehicle (e.g. that weirdly shortened nose on recent photos) or promises not kept by Elon (e.g. that this big honking single wiper won't be in the production version when it obviously is still there in the latest spy shots or (2) robot-generated negative comments meant to stunt Cybertruck sales - I wouldn't put that past a scared competition.
I personally have soured a bit on the Cybertruck - some because of the aforementioned ugly wiper, but mostly because Elon's "shine" has worn off quite a bit since he's revealed himself to be quite the a%$# from a political & human perspective. I bet Tesla will lose a lot of sales from his previously predominantly liberal base of fans.