Elon is probably mad it launched on an Ariane 5 and it went so perfectly, we’ll get an extra decade of science out of it.
The Ariane 5 really was a reliable rocket. It had some failures early on, like basically all rockets, but then it had 82 successful launches in a row and then one partial failure before having another long perfect streak.
Obviously, more expensive than modern reusable rockets but JWST was important enough of a payload, that I’m glad NASA/ESA chose Ariane. (Plus, given JWST’s delays, I imagine when that decision was made, SpaceX was still iterating and having occasional explosions.)