To my mind, a great songwriter is not defined as someone who works alone, or with a consistent partner. It's someone who writes songs that a LOT of people want to hear.
People like to hear, and re-hear songs that connect in some way to their own experiences. Taylor does that.
One of my favorites as always been "Everything has changed". It's a collaboration she wrote with Ed Sheeran way back in 2012. They wrote it together, at her house in Nashville. Ed has said Taylor brought most of the song and he contributed changes.
IMO, there's a lot to like. It's a touching story. A nice build to the music. Interesting rhythm changes, harmony, and catchy chorus. I could listen to most any day.
https://www.youtube.com/watch?v=w1oM3kQpXRo&list=RDw1oM3kQpXRo&start_radio=1
The people who wrote on 70% of her songs should be equally credited. But like Madonna and Mariah she puts her name on the creations of others for the royalty money.