Was the Montgomery bus boycott a turning point for the civil rights movement?

Absolutely it was. Not only was it the first time black people showed solidarity in their cause for civil rights, it was cripplingly expensive to the bus companies as most of their riders were black women taking the bus to mostly housekeeping-type jobs out in the "white" suburbs. White women, faced with having no domestic help that they were used to, were often reduced to going to collect their help, because the only alternative to the bus for many of the black women would be to walk. They had no cars. Whether the bus companies themselves applied political pressure I don't know, but faced with almost certain bankruptcy I would suspect they were lobbying long and hard.