In the context of what your granny said, she may have been trying to teach you a similar lesson. She may have been trying to tell you that not everything that seems appealing or attractive is necessarily good for you or in your best interests. It is important to look beyond the surface and consider the true value and consequences of something before making a decision.
For example, something that may seem glamorous or exciting on the outside may actually be harmful or dangerous. Similarly, someone who appears charming and friendly may not be trustworthy or have good intentions. It is important to be cautious and discerning and not to be fooled by superficial appearances.