Is it possible to make checkbox Enable Smart Cropping checked by default

You should be able to automatically enable smartcrop with the following code, added to your functions.php:

add_filter( 'wp_generate_attachment_metadata', array( $this, function( $metadata, $post_id ) {
	update_post_meta( $post_id, '_wpsmartcrop_enabled', true );
} ), 20, 2 );

I’m sorry, I missed a line in the code snippet. Try this:

add_filter( 'wp_generate_attachment_metadata', array( $this, function( $metadata, $post_id ) {
	update_post_meta( $post_id, '_wpsmartcrop_enabled', true );
	return $metadata;
} ), 20, 2 );

clearly I should stop trying to write code at three am… that was two different typos in two different snippets.

Try this:

add_filter( 'wp_generate_attachment_metadata', "smartcrop_enable_by_default", 20, 2 );
function smartcrop_enable_by_default( $metadata, $post_id ) {
	update_post_meta( $post_id, '_wpsmartcrop_enabled', true );
	return $metadata;
}