If you want a pop singer (or any kind of singer) with a very good voice - it may be that sacrifices must be made in terms of the... overall package. Sometimes the old phrase that "only a mother could love that face" (or whatever), springs to mind.
Of course there are some attractive singers who are very good singers (Debbie Harry from Blondie, for instance) - but good voices are rare enough that there is usually a trade-off between looks and voice.And this means that when the singer is less than gorgeous, it usually means that we are in for a vocal treat!
Typically, of course, pop singers are marketed on the basis of looks rather than vocal flair - and steps are taken during the recording process to cover-up the deficiencies. But when the vocalist is chosen by a musically-talented instrumentalists or composer; he is likely to choose a musically-talented singer.
The gifted composer and synthesiser virtuoso Vince Clarke exemplifies this with his picks; about whom we can say they are all very good singers, and also that you could tell they were going-to be good, even before they opened their mouths:
Note: Much the same applies in opera. My choice of probably the two greatest singers of the late 20th century would be Joan Sutherland and Luciano Pavarotti - and they certainly weren't picked on the basis of their looks: