It is an interesting and original suggestion that Ophelia might have been pretending to be mad. But what would have been her motive? And is she that devious? Or that talented? Would Shakespeare have decided to have her pretend to be mad without giving his audience some indication of what to expect? Hamlet is pretending to be mad, but Shakespeare has Hamlet warn the audience that he may put an "antic disposition" on, and elsewhere he tells his mother he is only faking madness and tells someone else that he is only mad when the wind is blowing from a certain direction and otherwise can tell a hawk from a handsaw.
Ophelia is an excellent example of an "ingenue." The word is borrowed from the French, of course, but it is in common English usage. Wikipedia defines it as follows:
The ingénue is a stock character in literature, film, and a role type in the theatre; generally a girl or a young woman who is endearingly innocent and wholesome.
A girl as innocent and wholesome as Ophelia would be highly unlikely to want to pretend to be mad. Even if she wanted to do so, she would have been incapable of doing such a good job of it. She must have really loved her father--even though the audience can probably see nothing particularly lovable about Polonius and Hamlet calls him a "tedious old fool." Polonius must have been a more dynamic and impressive character when he was younger. In the play he seems to be a victim of advancing senility. Ophelia probably loves her father because she is such a loving girl. She loves her brother too. She loves Hamlet. It is in her nature to be loving and uncritical.
Losing a father can be a shock to anybody. We see that Polonius is strict with his daughter. He does all her thinking for her, and she does everything he tells her to do. Her madness may be due to the fact that she suddenly has no men to guide her. Polonius is killed without warning, and her brother Laertes is far away in France, where it will take many days for word of his father's death to reach him and many days for him to travel back to Denmark. Early in the play Shakespeare shows us how Ophelia's father and brother are dominant forces in her life. Evidently he mother must have died. She has no one to turn to in her distress. What would have become of her if she hadn't died herself?
Shakespeare probably created Ophelia's madness and her death in order to heighten Laertes' motive to get revenge on Hamlet. Polonius is already dead and buried by the time Laertes makes it back to Denmark. With Ophelia's mad scenes, Shakespeare is able to dramatize the causes of Laertes' fury. His father has been killed by Hamlet, and Ophelia has become insane as a direct result. This makes it easier for the wily Claudius to talk Laertes into murdering Hamlet for him and thus solve the biggest problem that has been tormenting the king throughout the play. Claudius is an opportunist. Laertes' reaction to his sister's madness gives the king the idea of how he can use it to his own advantage.
O heat, dry up my brains! Tears seven times salt,
Burn out the sense and virtue of mine eye!(170)
By heaven, thy madness shall bepaid with weight,
Till our scale turn the beam. O rose of May!
Dear maid, kind sister, sweet Ophelia!
O heavens! is't possible a young maid's wits
Should be as mortal as an old man's life?(175)
Nature is fine in love, and where 'tis fine,
It sends some precious instance of itself
After the thing it loves.
Act IV Scene I