The problem you would run into is that there is no post ID for a new post and this would be the only way to get the post type.
If it is for an existing post the you could do
global $post;
$post_type = get_post_type($post->ID);
But like I said, for a new post this will not work because there is no post ID to get and if you get it you’re probably getting the wrong post ID.
In this case, I would add my prepare_field hooks directly to the template file where I am including the acf_form() call and have a different filter for each post type where I want to alter the labels and instructions so that I do not need to depend on the global $post value.
