That criticism has existed since Wide Sargasso Sea was published in 1966. Many critics of the time said that it could not stand alone, and that it had to be read with Jane Eyre to make complete sense. Early reviewer Walter Allen declared that the book could not exist in its own right. Rather than looking at Wide Sargasso Sea as a revenge novel, you could look at it as a novel of isolation, or as many critics see it, one that expounds of prejudice and race relations. I think that when you look at it this way, it becomes something that can stand on its own, and has much merit.
The above answer looks to literally at what is told within the text/narrative. i read this question as the extent to Rhys' motive when writing this -there's a quote saying she felt sorry for the portrayal of the madwoman in the attic, the ghost, and felt that she should give Bertha a life and a history - transform her from what EM Forster would have termed a flat character into a round one, i.e give her depth. Ultimately the novel is a work of revenge in that it gives a voice to what was oppressed in the work of Bronte, as in order to focus on Jane as the hero of the novel Bertha was demonised. The aspects of revenge therefore within WSS merely echo the wider metafictional level that Rhys was working on, during the 60's as a time of liberation and freedom. Aspects of the feminist movement can also be brought in here, see the unfavourable portrayal of Mr Rochestor when he is given his own first person narrative voice. WSS is a revenge on the colonial and imperialist repressive overtones that can be read in Jane Eyre, and adds another dimension of truth. See Salaman Rushdie - The Empire Writes Back.