Are these courgettes overcooked, if so how can I avoid it?

I think the biggest issue is you're baking them instead of roasting them. In order to roast them, you'll need a much higher temperature. 230c or 450f is about right. As Joe pointed out, the pieces are a bit thin for roasting. Spears or large chunks will serve you better. Roast them skin side down and brushing on a little olive oil will prevent the skins from drying out.

As in everything, you should keep an eye on what you are cooking; cooking is not an exact science.

Following timing instructions when baking/roasting is sometime problematic because each oven is slightly different and each ingredient will be slightly different from region to region (and season to season...).

So to answer the question, it depends.

If you feel they are overcooked, just cook them less next time or change the oven temperature or change recipe or cooking method.