As everyone always points out, Muffy continuously displayed highly selfish, snobbish, condescending and frankly disgusting behaviors over the course of the show (with the worst of it being in season 1 I think). There seems to be a pattern of traits she displayed consistent with NPD: 

1) Grandiose Sense of Self Importance   
Muffy consistently operated under the assumption that she was inherently superior to all her peers due to her family’s extreme wealth. She routinely demanded that ordinary spaces be bent to her high standards and acted like she was royalty rather than just another classmate. She always expected her friends to treat her as the centerpiece of their social circle.

2) A Profound Sense of Entitlement   
Muffy believed she was naturally deserving of special treatment, luxury and compliance from others. This sense of entitlement frequently pushed Francine to her breaking point in their friendship. She viewed luxury not as a privilege, but as a baseline of existence that others are obligated to maintain for her. When she faced even the slightest inconvenience, she expected the world to rearrange itself for her comfort believing the world existed to cater to her convenience. 

3) Inability to Recognize Others’ Needs  
Muffy was obviously tone-deaf to the economic realities of her friends. Because her baseline reality was one of extreme wealth and luxury, she completely lacked the empathy required to understand that her peers lived under vastly different financial constraints and could not simply buy their way out of their problems, afford expensive hobbies or match her lavish spending habits. She could not comprehend for the life of her why Arthur would spend time working tirelessly and saving every penny to buy a Moon Boots toy as she got everything she wanted cheap or expensive on demand. 

4) Interpersonally Exploitative Behavior  
Muffy regularly manipulated others for social gain. She had no issues using others’ hard work, talents, or emotional vulnerability to elevate her own self. She exploited Fern’s literary talents to sell poetry for her own profit, took credit for recipes she didn’t create to win a competition, and actively tried to sabotage The Brain to prevent him from winning “Student of the Month” so she could claim the title for herself. Muffy consistently treated her own friends as chess pieces to be moved, used, and cast aside for her personal elevation.

5) Excessive Need for Admiration  
Muffy could not tolerate fading into the background. Her desperation for continuous adulation often lead her to hijack platforms or steal praise from others. In How the Cookie Crumbles, she invited all her friends over of which they collectively created the recipe, but when the recipe won a major contest, her need for solo glory causes her to erase their contributions entirely and take all the credit for herself plastering only her face on the mass released cookie bags to bask in the towns praise solo. She is completely willing to alienate her closest friends if it means maintaining her image as a flawless success all by herself. 

6) Fragile Self-Esteem   
Muffy cannot tolerate failure, criticism, or being proven wrong, reacting to any blow to her reputation with immediate defense mechanisms such as denial, blame-shifting, and manipulation. When she got caught copying Francine’s answers during a math test, she immediately lied and let Francine take the entire blame and punishment rather than facing accountability just to keep her own reputation unblemished. 

7) Severe Lack of Empathy  
Muffy struggled to comprehend that her words and actions inflicted genuine emotional pain on her friends and justified her cruelty as just telling the truth. She frequently verbally tore down Francine and her other friends.  

Muffy had Narcissistic Personality Disorder