If Mark's gospel presents Jesus as servant, why does John say he's unworthy to loose his shoes? If Mark is the gospel which presents Jesus as servant, why did John the Baptist say in verse 7 that he was mightier than he and that he (John) was unworthy to stoop down and untie his shoes?
There is a difference between someone's worth, or nature, and their actions.

Jesus, in himself, is the Most High God* and worthy of our worship, praise and adoration. Nothing can change that. John is quite correct to say that  he is not worthy to untie his shoes.

In his incarnation Jesus takes on the role of a servant. It's a job he does, and it doesn't in any way diminish his nature.

You might consider the analogy of Prince William, who fulfilled the role of a (relatively junior) officer in the Royal Air Force. While fulfilling that role he accepted the orders of his senior officers, and interacted with other officers as an equal; despite the fact that in reality he outranked every one of them.

*According to most branches of Christianity.