To get rid of the indent, try editing the cell, placing the cursor in front of the text, and tapping delete.
Most text fields on the iPad will accept tab indent characters; but there is no way of adding them from the iPad's touch keyboard (that I know of). For instance, I can not tab in the Notes app on the iPad, but if I create a note with tabs in the Mac version and let it sync, the tabs are preserved. I can even select/copy/paste them.