Thoughts on Miku singing in English:
Intelligibility is important to me, but there's also a point where you have to respect a vocal synth for being what it is. Miku English is a vocal synth of a Japanese woman singing in English, which isn't her first language. That's not something you complain about; it's part of the color and part of the character.
I personally also find her struggles in this area thematically apropos. All of her users are struggling with something, whether it's with doing the music thing or something else. The fact that English takes a bit more work for her fits well into that idea--it's like there's something she struggles with, too. That shared struggle has been part of her character since "Miku Miku no Shiite Ageru."
That said, I wouldn't be opposed to an AI Miku who could do English more smoothly, as long as it sounds like her. There was nothing wrong with the direction of V4, IMO, for example.