It may not be the best name, but it sure is better than the names of the projects it was based on: PIK and FUIF. At least if you speak Dutch. A "pik" is a penis and a "fuif" is a party, so the combination would be the "penis party" image codec. I prefer "JPEG XL".
The etymology of the name "XL" is as follows: JPEG has called all its new standards since j2k something that starts with an X: XR, XT, XS (S for speed, since it is very fast and ultra-low-latency), and now XL. The L is supposed to mean Long term, since the goal is to make something that can replace the legacy JPEG and last as long as it did.
There might be an explanation around it, but I find it not much better than Apple with their iPhone XS. I agree that XL is a bad name, and I don’t think that “pik fuif” would matter a lot (I’m also Dutch).
Could have been e.g. JPEG PF or JPEG LTS.