Whether or not kissing your best friend on the lips is okay depends on the context and the nature of your relationship with your friend. Social norms and cultural factors can also influence what is considered acceptable in different situations.
In some cultures or social circles, kissing on the lips is a common form of greeting or display of affection among close friends, and it may be seen as completely acceptable. In other cultures or social circles, kissing on the lips between friends might be less common and could be interpreted differently.
The most important factor is consent and mutual comfort. If both you and your best friend are comfortable with the idea of sharing a kiss on the lips and there is no romantic or sexual intent behind it, then it might be okay within the context of your friendship. However, if either of you feels uncomfortable or uncertain about it, it's best to refrain from doing so to avoid any potential misunderstandings or awkwardness.
Communication is key. If you are considering this kind of gesture, have an open and honest conversation with your best friend about it to ensure that both of you are on the same page and understand each other's feelings and boundaries.
Remember that friendships can be complex, and what might be acceptable in one friendship may not be in another. Trust and respect are vital elements of any strong friendship, so it's essential to consider your friend's feelings and boundaries and proceed with sensitivity.