The key to selecting (or removing) an object from an image is to accurately trace its outline. This is especially important if the edge is jagged, has hairs, etc. (like plants & leaves) compared to smooth, well defined edges like cars.
Because of the small size of the screen, I have never been happy with the results when I have tried to accurately select an object on my cell phone or iPad. I don't remember all the mobile apps that I have tried, but they certainly include "PS Touch", "PS Express", and several others. If you are going to do this sort of work, I would suggest a desktop machine with a decent sized monitor and stylus (ie, not a mouse, or attempting to move the cursor around with the end of your finger, as on a phone).