Is a soliloquy always a truthful representation of a character's thoughts?

Not always.

A soliloquy is a type of monologue often used in drama and is used to when a character addresses other characters, the audience or most oftentimes--himself. This device is used to relate the characters thoughts and feelings expressed like unspoken reflections. Thus, it most the times truthful but can be deceptive.

A soliloquy is a speech that is delivered while the speaker is alone. It reveals what the character is thinking, rather than what the character wishes to say out loud to the other characters. For that reason, it truthfully expresses the character's thoughts. PLATO