She was alright. Hotter than Yoko, anyway, tho that is purely a subjective opinion.
Cynthia was the neighborhood girl that got knocked up, not much more. She loved John and tried to make things work, but neither of them were ready for what it took to be a Beatle (hell, the world wasn't either). He never was 'in love' with her, theirs was a typical arrangement of parentage for an out-of-wedlock child.
Yoko was indeed more of a spiritual and mental challenge for him, and she did catch him when he was in his most vulnerable period. Still, he seemed attracted to her all his life so for whatever reason(s) she appealed to him where Cyn didn't.
But don't sell Cynthia short, she was just every-girl who turned into every-wife yet was caught up in the maelstrom of the 60's and specifically the Fab's existence.