Why did my lemon yoghurt cake turn out bitter? I used zest but made sure the white pith did not go in.

What kind of curd did you use? In my experience over zesting will cause a tart bite but it takes a lot of extra. Over cooking your zest will also add a bitter flavor. If you post your recipe and major cooking steps it would be easier to identify the problem.

Were the lemons waxed or unwaxed? If they were not unwaxed they would have been coated with wax and preservatives, some of which can taste very bitter and leave a burning taste in your mouth.

Which recipe? I'm wondering if the am't of baking powder was somehow to blame?