This miscommunication and lack of communication play a bigger role in Mansfield Park than good communication. Whether communicating verbally or non-verbally, characters are frequently misunderstood, fail to make themselves clear, and misunderstand others. People assume things and hear what they want to hear. Sometimes they have to fill in the blanks, though. Silence is often a popular "communication" choice for characters who want to keep a secret or are scared to draw attention to themselves. But silence often speaks louder than words, so to speak, and silence is almost always much more inaccurate than words. However faulty language may be, silence often causes more problems.
Fanny's silences, utilized to guard her secrets and to protect herself, just end up inviting people to make faulty assumptions about her, which causes more problems than speaking up would have caused.
If Fanny talks too little, then Mary talks too much and without any discretion or caution.