You need to return a value, not echo it. As the docs for add_shortcode() state (emphasis mine):
Note that the function called by the shortcode should never produce output of any kind. Shortcode functions should return the text that is to be used to replace the shortcode. Producing the output directly will lead to unexpected results. This is similar to the way filter functions should behave, in that they should not produce expected side effects from the call, since you cannot control when and where they are called from.
Try:
add_shortcode('mn','mn_func');
function mn_func($atts){
return "<p> Testing </p>";
}