It can have a negative connotation, but in my experience it depends on the context it’s used in.
Here a Star Trek pastiche would mean something outside Star Trek that has a look and feel of Star Trek, which would not be negative as long is it’s done well. The Orville in my opinion has both the pastiche of a Star Trek (specifically TNG) series, and the substance, with enough of it’s own style to not be purely derivative.
On the other hand a Sci-Fi pastiche would imply something taking elements of the genre without managing to be in the genre, it implies some level of failure or inauthenticity. I can’t think of an example of this for Sci-Fi, but you see it often with Film Noir where someone does some stark lighting, films in black and white and adds a film grain effect and calls it Film Noir without any of the story trappings that defined the genre, that is the pastiche without the substance.
