Can I used lemon juice in place of ascorbic acid in canning peaches?
No, you should never make substitutions in canning recipes. 

You are right that the ascorbic acid is in there for preservation rather than taste. But the problem is that with a lemon, you can't controll how much acid ends up in your peaches. Some lemons will contain more grams of acid per ml of juice than others, and in any case, you'll need really large amounts of the juice, to the point where you'll have a very pronounced lemon taste. 

Don't forget that the pH scale is logarithmic, so a peach preserve at ph of 3.5 will have several times more acid than a preserve at a pH of 4.0. And botulism bacteria which will die at home canning temperatures in a pH 4.0 medium can survive at pH 3.5. So adding some juice and hoping that it will be enough is a very dangerous strategy. 

If you are making a small amount of preserve only, you can consider keeping it in the fridge and consuming it within a week, instead of canning it with acid.