I know this is a minor detail, but this is Edgar Wright. Come on. When Baby and Debora are talking at the diner, he tells her about the song based on her name, but he calls the artist Trex instead of T. Rex, as corrected by Debora. Is there any symbolism or hint of a subplot here? Maybe he has dyslexia or trouble reading...? Is this ever hinted at anywhere else? Thanks.
No. I suspect you're overthinking this.
Baby just isn't aware of the proper way of saying the band's name.
After all, the band dissolved 30 years (when Bolan passed) before the movie timeframe. Baby wasn't even born.
Apparently Edgar Wright mentions in the DVD commentary that..
A studio rep asked him to cut the line where Baby refers to the band T-Rex as Trex because they didn’t want him to seem dumb. He refused as it reads endearing not dumb.