sounds like my mother
Since Lucid Dreaming isn't so popular(I just came to know about it last month), people (like my mom) tend to associate this stuff with paranoia or stuff like that.

Don't try to explain, I don't think she really wants to hear your reasons.
Though you can try saying that it will help your concentration and memory (hey, you improve your dream recall, isn't that better memory? and there are various techniques as well where you have to concentrate, so no lying here! ).
Say that Einstein used to do this too! (I've "just heard" about it though, I can't say its true)
If she still doesn't listen, say that you have stopped doing it, I know its lying and its not good, but isn't it still better than to have an argument with your mom everyday?
Remember, you have the right to do anything you want, don't let your mom hold you back, its your life, live it your way.