There’s no way of selecting all text frames in ID, and even with scripting it’s not possible.
Of course, with scripting you can easily create a collection of text frames and do things to that collection.
However, if all you’re needing is to move all text frames in a document the same distance up or down, left or right, you’ve got at least 2 options:
(1) Before doing the datamerge, make sure that you’ve applied an object style to the placeholders. The same object style to all placeholders. Now do the datamerge.
At this point (presuming you’re using a newish version of InDesign), you can actually adjust the position of all objects that have that style applied in the entire document by modifying the object style “size and position” properties. Just edit the object style and they’re there, 4th or 5th from the top.
Does that help?