jQuery is not defined in Wordpress, but my script is enqueued properly

You can try:

enqueue Jquery first.

wp_enqueue_script('jquery'); 

and then enqueuing the latter script with JQuery dependency in 3rd argument i.e array('jquery') that's what mostly people forget.

wp_enqueue_script( 'lapetitefrog-mobile-menu', get_template_directory_uri() . '/js/mobile-menu.js', array('jquery'), '1.0', true );

First Make sure that jquery file is include in the header, and your file requied jQuery

wp_enqueue_script( 
        'lapetitefrog-mobile-menu', 
        get_template_directory_uri() . '/js/mobile-menu.js', 
        array('jquery'), 
        '1.0', 
        true 
);

Second you should start your javascript file like:

(function($) {
    $(document).ready(function() {
        .......
    });
})(jQuery);

OR

// Use jQuery in place of $
jQuery(document).ready(function() {
    .....
});

Add wp_enqueue_script('jquery'); before you enqueue your scripts.