Can ghee replace butter in lemon curd?

Yes, you can; for example, from Charlotte's Lively Kitchen we have the following remark:

Can lemon curd be made with clarified butter?

OK, this is maybe a slightly unusual recipe adjustment to have tested. However, I decided that as lemon curd is such a classic recipe, I’d see how it was made in an old Victorian cookbook I was given by my Granny, and that recipe used clarified butter so I thought I’d give it a try.

It was delicious. However, I didn’t think it tasted like lemon curd as we know it today so I decided to stick with regular butter.

Note that butter is about 80% fat, 18% water, and 2% milk solids (varies according to the butter). So, you may wish to reduce the amount of ghee and adjust the moisture content based on your recipe.