If the cream is too warm when you start, the fat becomes ineffective as a stabilizer, and the whipping cream falls flat. You will not be able to whip it into peaks, and the pie will not be as lofty or light. (It will still firm up as it chills, so all is not lost.) Both the friction of the mixer whisk and the temperature of the strawberries will warm the cream as you mix it, so you want everything start out as cold as possible. (Including the mixing bowl!)