If the goal is defacing a position, the phenomenon in question may be a type of strawmanning, perhaps more specifically strawpuppeteering. From Wikipedia:
A strawman sockpuppet (sometimes abbreviated as strawpuppet) is a false flag pseudonym created to make a particular point of view look foolish or unwholesome in order to generate negative sentiment against it. Strawman sockpuppets typically behave in an unintelligent, uninformed, or bigoted manner, advancing "straw man" arguments that their puppeteers can easily refute. The intended effect is to discredit more rational arguments made for the same position.
If the goal is making the cause appear as already being attended (ie. bystander effect), or perhaps appear as lost or futile, concern troll or "Do-Nothing" may be more fitting. From Wikipedia:
A concern troll is a false-flag pseudonym created by a user whose actual point of view is opposed to the one that the troll claims to hold. The concern troll posts in web forums devoted to its declared point of view and attempts to sway the group's actions or opinions while claiming to share their goals, but with professed "concerns". The goal is to sow fear, uncertainty, and doubt [FUD] within the group [...]
Wikipedia has it James Wolcott quotes Saul Alinsky as saying:
[At least some] Do-Nothings profess a commitment to social change for ideals of justice, equality, and opportunity, and then abstain from and discourage all effective action for change. They are known by their brand, 'I agree with your ends but not your means'.