When making homemade soap is it bad to just directly add lemon juice?
You would have to use the lemon juice in place of some of the water and either add the lye to the water/lemon juice solution or hold the lemon juice out and add it to the soap batter once it reaches a light trace.
As I've never tried this, I'm not sure what the result will be. I suggest experimenting with a small test batch and be sure to take lots of notes.
Be aware that the lemon juice will not really give the soap a 'lemony' scent. For that you will need an essential or fragrance oil.