Cream cheese makes for a suitable substitute for the mascarpone in this recipe. One thing to keep in mind is that cream cheese includes more salt and is generally slightly more acidic than mascarpone. You may need to slightly reduce the amount of mentaiko and just the amount of lemon juice if you decide to use cream cheese. It is also much firmer than mascarpone, so you'll want to bring it up to room temperature to be able to mix it. Sour cream can also be a suitable substitute, but you will want to omit the lemon juice and just use the zest.