Yes, you can do that using wpForo hooks. Put this code in your current active WordPress theme functions.php file. You can change the length too. I set it 100 for you.

function wpforo_custom_notification_text_length( $length ){
   $length = 100;
   return $length;
}
add_filter('wpforo_notification_saved_description_length', 'wpforo_custom_notification_text_length', 10);
add_filter('wpforo_notification_description_length', 'wpforo_custom_notification_text_length', 10);

How to Easily Add Custom Code in WordPress (without Breaking Your Site)

Currently, when someone posts something new on a topic I am subscribed to, I get a notification containing the post topic name and one sentence from the reply. Is there some way to display more of the reply? Right now I believe it shows something like 50 characters. I'd like to increase that. Is that possible?

For example:

Hello! New reply has been posted on your subscribed topic -

In California, all new staff are currently mostly working from home. Our workers and buildings are...