No, it is a single, specific event of 'being polite' - it occurred 'yesterday', a well specified and bounded time. Passé composé is correct here - there is a very definite and distinct event being described. It is not suggesting an ongoing, repeated or habitual behaviour. 

In French, and in English, such a sentence suggests that 'being polite yesterday' was noteworthy for being unusual.

If it was just "you were polite" (no time specification), or "you were polite when you were young", or "when you wanted something" - imparfait would be appropriate.

Although < être > is used in imparfait frequently, it is not restricted to imparfait.