I cannot resist adding Jessye Norman here, because I think that she can indeed be described beautiful, also. Perhaps impressive would be more accurate, but she really knows how to carry her weight with authority. And with a voice like hers, what else matters?
Not necessarily. But mention the words 'pretty' and 'classical musician/singer' in the same sentence and people will all of a sudden act as though the two don't go together. It's as though it's a crime to be pretty or attractive and we're only allowed to take classical artists seriously when the're fat, bald, ugly and have a look on their face as though they never had a day of fun in their whole life......I'm exaggerating, but you know what I mean.
Besides, in lieder, oratorios (or opera cd's) and such it doesn't matter whatsoever how a singer looks. But although opera is first and foremost also about the singing, it's not the only important ingredient. Opera is also a form of theatre. And just like in regular theatre or the movies it's important to cast people who look the part. This should never be the most important consideration, but it shouldn't be entirely ignored either. Maria Callas herself lost weight and improved her physical appearance because she realized that it would be appropriate for many of the roles she sang.
Martha doesn't signal when the orchestra comes in, she's just pursing her lips..
How did I ever forget the ravishingly exotic Danielle de Niese? Of Sri Lankan heritage de Niese is not only a talented singer but she is also a fine dancer and can act (something a good number of operatic singers are not so good at) and exudes pesonality:
de NIese was absolutely marvelous... and stunning as Hebe in William Christie's production of Rameau's Les indes galantes:
At the end of the opera Hebe/de Niese acts as something of the "ringleader"/master of ceremonies: