Yes, there have been very great singers with less than beautiful voices, and there have been even more singers with beautiful voices, who, unfortunately, are extremely uninteresting musicians and performers. Much depends on the repertoire. Mozart sung by unbeautiful voices is usually intolerable, but in other repertoire power or agility are more important than a beautiful voice, and even in Mozart good technique is more important than lovely timbre. A beautiful voice is a wonderful thing to have, but it isn't the most important thing.
Ballet is not necessarily that different. A beautiful physique is also a wonderful thing, but it doesn't automatically translate into a good dancer. Just as I've heard lots of lovely-sounding but boring singers, I've seen (so-called) dancers with beautiful bodies, who are also awkward, technically deficient, unmusical, inexpressive, dramatically inert and/or just plain uninteresting. And as with opera, some ballets can tolerate flawed bodies better than others, as yudi noted.