Why is my voice monotonous / too chaotic / doesn't sound similar etc?

Try changing your voice settings, you'll find them in the "Voice Settings" tab. Each attempt to generate a voice will bring a different result (especially noticeable at low stability settings).  You can find out more in our guide to voice settings.

The performance of the voice is also highly dependent on the voice itself; if you are using a pre-made or voice-designed voice, they tend to be more tempered and less emotional, while a cloned voice tends to exhibit a wider emotional range but that is also highly dependent on the original voice used for the clone.