it was

If you refer to a single, isolated event in the past, the passé composé is the appropriate tense. And a lightning strike (coup de foudre) is a pretty well isolated event rather than a state of being.

You will see the figurative phrase in passé simple quite regularly "Ce fut le coup de foudre". You will also find examples of "C'était le coup de foudre" - the phrase being figurative for 'love at first sight', and imparfait possibly being used to indicate the 'love' commenced at a figurative 'first sight', and was ongoing. Whether that is grammatically correct or not I will leave to others to debate.