How significant was the Montgomery Bus Boycott? I am struggling to find anything that says it was insignificant.
I don't think very many people would say it was insignificant, which is probably why you're having trouble finding any such sources. But it is important to understand that the bus boycott was not absolutely vital to the civil rights movement, which would have carried on without it. The Montgomery Bus Boycott was one of many protests and sit-ins that contributed to the progress of civil rights in the US; any particular protest could have been removed and the whole would have survived, but remove them all and the movement would have failed. (There is a Chinese proverb I find very insightful on this sort of thing: "No raindrop feels responsible for the flood.")
Rosa Parks initiated the bus boycott quite intentionally; she was not simply a random person fed up with the system as she is often depicted, but a dedicated civil rights activist who was active in the NAACP. Her refusal to sit in the back was not an impromptu decision, but a planned act of civil disobedience.
One thing that was important about the bus boycott was that it triggered the formation of the Montgomery Improvement Association, which elected as its leader a 26-year-old pastor and activist thus far largely unknown: Rev. Dr. Martin Luther King, Jr. I'm guessing you've heard of him.
If not for the bus boycott, it's hard to say whether Martin Luther King would have become the prominent and world-renowned activist leader he ultimately did. My suspicion is that he would have, that some other cause would have arisen for him to lead even if the bus boycott hadn't happened. But it's also possible that MLK himself would have remained unimportant, and other civil rights leaders would have stepped up instead. One thing is certain: The civil rights movement in the US would have carried on regardless.
Activism for the civil rights of African-Americans and others has gone on for hundreds of years, with periods of progress and periods of failure along the way. Rosa Parks and Martin Luther King were important in bringing civil rights issues to public attention in the 1950s, but there were thousands or even millions of other people fighting for the cause.