Can I use fresh lemon juice?
I don't really recommend it as the acidity can cause the buttercream to split quite easily. If you want to use fresh lemons then I'd recommend adding lemon zest instead. The bitterness of the zest helps to cut through the sweetness a little and won't cause splitting. Make sure to grate it finely before adding it and use a large piping tip when piping- it may get stuck in the tip otherwise.