The problem for all those people in that commercial mis-singing "Rocket Man" isn't so much that Elton John is some kind of mushmouth, it's that he and Bernie Taupin can't get on the same page as far as which syllables to emphasize. In the line "Burnin' up his fuse out there alone," EJ's music stresses the syllable "there," which isn't the worst possible syllable to emphasize but is clearly in the bottom half. And then the music crams the entire two-syllable word "alone" into not much more than a single note.
One song that makes the words almost impossible to understand - although nobody cares, because it's such a fantastic song - is the Jackson 5's "I Want You Back." In the chorus, the first half of each line is punched out like Regis is singing it, only to force poor Michael to get four or five words onto a single note in the second half: "Won't! You! Please! Let! Me! Backinyourheart." Michael kinda survives that one, but if you say you understood "Now! That! I! See! Youinhisarms" the first time you heard the song, you're lying.
You know, those people in the TV commercial wouldn't need whatever product that ad is pushing if they'd just listen to the Shatner version. Among the man's many virtues, he sure can enunciate: