Add disclaimer before certain posts and pages

Is there a better way to accomplish this using the affiliate tags on
my posts?

Definitely I would go for an easier option, not relying on plugins like Jetpack.

Why don’t you try the has_tag function? Something like this:

function tt_filter_the_content( $content ) {
    if (has_tag('affiliate'))
    $custom_content="YOUR MESSAGE";
    $custom_content .= $content;
    return $custom_content;
}
add_filter( 'the_content', 'tt_filter_the_content' );

This will filter your content and add your “YOUR MESSAGE” on all posts with the “affiliate” tag name.

Reference https://developer.wordpress.org/reference/functions/has_tag/