I was going to suggest this too, an obvious choice maybe but iconic and what a song You're On Your Own Kid is! I am a devoted Swiftie, named our daughter after one of hersongs, not Betty, Marjorie, Dorothea, Clara, Cassandra, Aimee or Emma though!
In my opinion, she's the greatest lyricist in pop music history, a true poet, so narrowing down favourite lyrics is hard but your suggestion is a great one. You're On your Own Kid is also the song the newly released and long-awaited Eras snow globe plays so it clearly has significance to Taylor herself. The problem is that some of my favourite Taylor lyrics are quite melancholy, so probably not ideal choices for a piece of hopefully uplifting wall art! However, here are a few other favourites of mine that might fit the bill:
"Best Believe I'm Still Bejewelled when I walk in the room
I can still make the whole place Shimmer."
or maybe
"What's a girl gonna do?
A diamond's gotta shine."
both from Bejewelled on Midnights which is a pretty positive song
If she's into cats, like Taylor herself, then these lines from Karma might be fun!:
"Karma is a cat
Purring in my lap cause it loves me."
This beautiful lyric from Paris, also from Midnights, would make for a great artwork I reckon:
"Levitate above all the messes made
Sit quiet by my side in the shade
And not the kind that's thrown
I mean, the kind under where a tree has grown"
happiness from evermore is an exquisite song with some of her best lyrics, which is saying something, but most of the lyrics are very sad and wistful. However, I have always thought there is a lot of hope reflected in the lyrics too, and this lovely lyric and is a great example of that and would probably very much appeal to a die-hard Swiftie like your daughter as this is something of a fan favourite:
""In our History
Across Our Great Divide
There is a glorious sunrise
Dappled with the flickers of light
From the dress I wore at midnight.
Leave it all behind."
There are also two lines from tracks on 1989 which, while not amongst my absolute favourites, I always thought were fun and pithy and would make great t-shirt slogans. First, from Blank Space:
"Darling, I'm a nightmare
Dressed like a daydream."
From New Romantics:
We're too busy dancing to get knocked off our feet.""
If I were you I'd probably play it safe and go for the You're On your Own Kid lyrics, but I'd personally choose the happiness or Paris lyrics as they are my favourites! Good luck, I'm sure she'll love whatever you choose!