I can't believe how rude people are being to the OP, over both the term 'nickname' and the one she has chosen.
To all those insisting that Lottie can only be short for Charlotte - why are you saying it should be Lettie instead? That's an established short for Letitia. In your world, would Lettie not be 'disallowed' too? And I take it you wouldn't call your little Edwards Teddy, as that can only be short for Theodore? Lighten up, people. Short forms can be short for more than one thing.
As others have pointed out, lots of short forms are a step or two away from the original but it doesn't make them any less valid.
But is it a 'nickname'? I think that's the American term for short forms (I'm sure I'll be corrected if not!) and until recently I'd never heard it used in the UK in that sense. A nickname always used to be something like Titch, Squidgy, Pongo, Tubs, etc. Short forms were always just called 'for short'. The proper word is hypocoristic, although it sounds a bit pretentious to use it!
So in the sense above, you can't choose a nickname in advance, as they evolve themselves. But you can choose a hypocoristic, and if the chosen name has a lot of short options e.g. Elisabeth, then deciding which you'd prefer in advance is not silly.
Go for it, OP, and ignore the naysayers!