Note the exchange between Prospero and Miranda Act I scene ii l 473f: 'Thou thinkst there is no more such shapes as he [Ferdinand],/Having seen but him and Caliban...'.  Earlier in the scene (l 308), when Prospero says 'We'll visit Caliban my slave', Miranda responds with ''Tis a villain, sir, I do not love to look on.'

Miranda's reference in Act V to 'goodly creatures' would not seem to include Caliban, perhaps because he is not 'beauteous' or 'goodly, and (or?) because he is not of 'mankind'.  But Prospero numbers Caliban among the 'shapes' on the island, one Miranda has seen and has not deemed wondrous.  So while she may be wondrous about the people who are new to her, she knows not all 'shapes' on her island are equally so.  Perhaps she is rejoicing that her new acquaintences are not as unlovable to look on as her oldest - which indicates less than complete innocence.  In which case Prospero's remark about the island 'It is new to you' contains a deeper irony.

Prospero's line, "'Tis new to thee," might have gotten a lot of laughter from Shakespeare's audiences, especially from those who had lived long enough to gain experience of mankind. Miranda is young and has "great expectations." Prospero is old and knows what to expect from humanity. Maybe both perspectives are valid. Young people like Miranda are still experiencing the wonder of existence, including the possibility of love. Old people have experienced innumerable disappointments and painful experiences. They have also realized the worst truth about life, which is that everyone is condemned to die and lose everything, including any loved ones they might be leaving behind. When Prospero tells his daughter "'Tis new to you" he is stating a simple truth: the world is everything she says it is when one is young. He doesn't go on to say anything such as, "You'll feel differently when you get older," because he knows it would be useless and that there is no point in trying to disillusion the young. Prospero is almost talking to himself when he says "'Tis new to you," and no doubt Miranda scarely hears him. With the arrival of the survivors from the tempest, Prospero's island becomes a microcosm of the real world, with all kinds of humanity represented.

In Shakespeare's The Tempest, Prospero, robbed of his title as Duke of Milan, knows the way of the world. Antonio and Alonso treacherously set Prospero and his young daughter (Miranda) adrift on a small, leaky boat without a sail. They only survive because Gonzalo loads the boat with provisions for their survival. Finding the island, Prospero and Miranda have survived and thrived. But Miranda knows nothing of mankind, for she has not been exposed to its evils.

At the outset, there is a terrible storm that shipwrecks Prospero's enemies on his island, scattering them all in different locations. In Act one, scene two, aware of her father's magical powers, Miranda asks him to end the storm, fearful over the fate of the ship's passengers and crew. Prospero comforts his daughter, saying none are lost—and all he does, he does for her welfare. He notes that there is more than what she knows of herself, and more to her father than she could imagine:

PROSP:
No harm.
I have done nothing but in care of thee,
Of thee, my dear one, thee, my daughter, who
Art ignorant of what thou art, naught knowing
Of whence I am, nor that I am more better
Than Prospero, master of a full poor cell,
And thy no greater father. (18-24)

Miranda assures Prospero that she need not know more. Prospero admits that while he hesitated before to share her real story, he says the time is right. (He must tell because his enemies have arrived—life will be changing). Prospero tells her that he is the Duke of Milan, and that she is a princess. He also says that being brought to the island was both a a curse (because of the treachery practiced on them), and a blessing—because they were saved by arriving there. We (as the audience) can infer that they have also been happy.

MIRAN:
O, the heavens!
What foul play had we that we came from thence?
Or blessèd was't we did?
PROSP:
Both, both, my girl.
By foul play, as thou sayst, were we heaved thence,
But blessedly holp hither. (72-77)

While Prospero (with the help of Ariel, an "airy spirit") manages the activities and damage his enemies are able to accomplish on the island, Miranda meets Ferdinand, Alonso's son and the Prince of Naples. The two fall in love. Miranda's experience throughout the play has been limited in that she has not seen the darkness that drives some men souls, as was the case with Alonso and Antonio. (However, by the end, both men have repented of their "sins" toward Prospero, returning his title to him.)

Alonso believes Ferdinand died—lost in the sea. In Act Five, scene one, when he finds his son playing chess with Miranda, can hardly believe what he sees.

ALONSO:
Now all the blessings
Of a glad father compass thee about! (204-205)

Miranda loves the good Ferdinand; now she sees his humbled and joyful parent rejoicing in that his son is alive. She is impressed by this parent's love. Miranda's exposure to the human race is very limited, and her new experiences have only shown her the better side of humankind, so she exclaims:

O wonder!
How many goodly creatures are there here!
How beauteous mankind is! O brave new world
That has such people in't! (207-210)

Prospero's response is based upon Miranda's lack of experience when he notes:

’Tis new to thee. (211)

He could be saying that she would naturally think the world is beautiful because she has not seen many men, and no men at their worst as Prospero has, or (maybe) that beyond this limited scope, there are men even more wonderful.