Incorrect. It's a matter of defining your context and audience.
In context: "Mainstream" is the broad group in which sub-groups may exist. "mainstream subgroup" is a contradiction in terms.
"Star Trek fans" are a subset of science fiction fans (and arguably, intermediate of that, fans of space opera), and science fiction fans are a subset of fiction readers/viewers. The "mainstream" refers to that which appeals to the broadest base of the superset and implies that what will be true for the mainstream will usually be true of an equal proportion of the sub-groups.
A group that deviates significantly from the mainstream is referred to as a "niche" or a "cult." Such a group is not considered part of the mainstream because it does not reflect mainstream trends in some important way.
To be perfectly clear on this point:
Is correct
JJ Trek fans are a subset of Trek fans.
Is incorrect.
JJ Trek DETRACTORS are a subset of Trek fans.
Thank you for giving me the Venn Diagram idea, because it works out roughly like this: