The speaker of the poem asks the girl to look at the "prospect." Here, prospect can mean the view but it also can mean the probability or chance for success in the future. So, it is even more tragic that the child cannot see a chance for success in the future.
In the late 18th and early 19th centuries, the blind would have an easier time adapting than the deaf and/or mute; however, their prospects were certainly not as good as those of sighted people. The girl may end up in an institution, she may end up a beggar, or she may find some success in a "normal" life. A large part of her ability to succeed would have to do with her family's income and ability to help her. The better off they are, the better chance she would have.
In England, schools for the blind began in 1819, close to the end of Lamb's life. Braille was first published in 1829. The poem is meant to be melancholy, and seems to suggest that the girl has no "prospects." It is true that she would face an uphill battle. The poem was written before braille and schools for the blind were established. Had it been written a few decades later, maybe Lamb would have given it at least a glimmer of hope. That being said, his intention in writing it is also to evoke sympathy for the blind and therefore he is suggesting that there should be means of helping the blind to have better life prospects.