The band was slowing down and the members were wanting to move on to their own careers. They were no longer happy, they were fighting a lot, and it was time to move on. The Beatles were a fad and that fad went out of "style." They were already over and they were tired.
No Yoko Ono was not the only significant other that was a part of the Beatles. I feel that like most bands, they would have broke up either way within a matter of time. It is a known fact that music groups are only good for a short period of time. Bands break up all the time because of personal differences and I am sure that it was not Yoko Ono that broke the band up.
According to an interview with Yoko Ono, she states that each of the band members expressed feelings of wanting to leave, with the exception of Paul McCartney. It was simply a matter of when it would happen. All bands eventually break up, typically due to resentment or personality differences. If one person has more of the spotlight, the others in the band eventually grow resentful of that band member.
No, I do not agree that Yoko Ono is responsible for the break up of the Beatles. I think that the Beatles would have eventually fell apart regardless if she were a significant other of John Lennon or not. Other members of the Beatles had significant others and they are not blamed. Why blame Yoko Ono?