Is it mandatory to use contractions in tag questions and the like?

It’s just extremely common to see tag questions that use contractions as opposed to their uncontracted forms.

A COCA search for , _v* * * ? (comma, verb, two words, and a question mark; note that “n’t” is considered a separate word) shows that “isn’t it?“ is much more common (5395 hits) than “is it not?” (334 hits).

Still, 334 hits is not insignificant. There’s nothing wrong grammatically with the uncontracted form. It (in general as well) is just more emphatic and sounds more formal.

Tag's generally use less formal language as a standard because tags themselves are a less formal way of speaking.

You could use the full form if you want, but the tone changes and makes the sentence more emphatic. Sometimes this change in tone is what you're intending.

For example:

You knew you were coming home late, didn't you?

... sounds less severe than...

You knew you were coming home late, did you not?

... which sounds like something an angry parent would say to emphasis the gravity of the situation.