A mug with a daft face has a strong advantage over a perfectly sensible mug. Both can hold a hot drink, but only one looks faintly surprised to have been given the job. That is a useful contribution to an ordinary morning, especially if its expression becomes more appropriate as the tea disappears.
I like an object that looks as though it has a small personality. A slightly wonky smile, enormous painted eyes, or a handle that turns the whole thing into an animal. It does not need an explanation. It can just sit on the desk looking pleased with itself while doing the very respectable work of being a cup.
There is a whole imaginary office politics to a shelf of these things. The serious mug, the cheerful mug, the mug that appears to know a secret. Put them beside each other and it is remarkably easy to assign roles. The largest one is clearly in charge of refreshments, though its management style might leave something to be desired.
A comfortable handle and enough room for the drink still matter. After that, I am willing to give points for making the cupboard more entertaining. If a little ceramic creature can improve a tea break simply by looking ridiculous, I think it has understood the assignment.