The level of detail in your mock-ups should reflect the level of confidence in your design

Mock-ups are prepared as a way of exploring a particular design problem, and in order to communicate your thoughts on a solution to important stakeholders (i.e. users, clients or developers).
In the early stages of design, mock-ups should communicate broad concepts and groupings of information or functions. Keeping the details scant at these early stages allows you to progress through iterations quickly and keeps your stakeholders from throwing the baby out with the bath-water by quibbling over colours and button placement.

As you progress towards a final design, mock-ups should be rendered in increasingly higher fidelity so that the full detail of your approach is exposed.