Sera being taller and stronger looking than your average elf could be put down to the fact that she was adopted by a noble woman at an early age and so would have had a far better diet than the majority of alienage elves. Dalish might have a more healthy diet in comparison with city elves but the harshness of their existence would also mean they would probably not develop as they would if they had an easier life. I think the ancient elves are meant to be taller looking than modern elves but they didn't seem to carry this through with Solas. In any case, among any race or species you will have exceptions to the norm.
The lack of variety in NPCs is probably down to time and resources. If they spent these on populating their world with more realistic variety of people, this would detract from some other area of development.
Mind you, it may be that they don't have people being born with hereditary blindness, deafness or other disabilities. There is no mention of this in the source books, so perhaps it is not a feature of this world, or they decided it was best not to bring it up or people would expect to see examples in game.