It's not as useful as it sounds... you have to know how to adjust for various physics issues that prevent banks from moving on a perfect mirror line. And figuring out that line is easy without an app.
Even so, I'd consider it cheating. The 'spirit of the rules' seems to be that you cannot use outside aids for aiming. For example you can't lay your stick on the table to estimate a bank (though it's ok if you keep a grip on it). You can't mark the cloth. And in some leagues, you can't rest chalk on the rail or get outside help from another player (except when the league allows timeouts).
If I can't consult another person on how to play, I shouldn't be able to consult the app.
I might be tempted to let it slide knowing that the app is of limited use, but if it helps with 2 and 3 rail kicks... that's actually too useful to ignore. Finding the aiming point for these (prior to 'physics adjustments') is not that easy or well-known. I'd feel cheated if an opponent got the solution for that sort of kick from an external source.