Mountains in the Bible represent - Ruling governments, rulers which can be bound- up and cast into the sea!
The fig tree and demon both represent obstacles "mountains- one does not produce what's needed and the other destroys God's handiwork.  Both can be uprooted - and cast out into the sea.
Both are termed "mountains" that can be removed.

I can guarantee this truth: This is what will be done for someone who doesn't doubt but believes what he says will happen: He can say to this mountain, 'Be uprooted and thrown into the sea,' and it will be done for him. That's why I tell you to have faith that you have already received whatever you pray for, and it will be yours.
Mark 11:23,24

 "Look, O mighty MOUNTAIN, (Babylon) destroyer of the earth! I am your enemy," says the LORD. "I will raise my fist against you, to knock you down from the heights. When I am finished, you will be nothing but a heap of burnt rubble. Jeremiah 51:25

The disciples were surprised to see this. They asked, "How did the fig tree dry up so quickly?" Jesus answered them, "I can guarantee this truth: If you have faith and do not doubt, you will be able to do what I did to the fig tree. You could also say to THIS MOUNTAIN, 'Be uprooted and thrown into the sea,' and it will happen. Have faith that you will receive whatever you ask for in prayer." Matthew 21:21,22

Then the disciples came to Jesus privately, and said, "Why weren't we able to cast it (demon) out?" He said to them, "Because of your unbelief. For most certainly I tell you, if you have faith as a grain of mustard seed, you will tell THIS MOUNTAIN, 'Move from here to there,' and it will move; and nothing will be impossible for you. But this kind doesn't go out except by prayer and fasting."  Matthew 17:19-21

Demons represent the kingdom or rule of Satan - they are "mountains" or obstacles that can be bound, removed and cast into the sea. It is not a faith which is needed to cast a literal pile of rocks or "mountain" out and into the sea. It is a faith which binds-up and cast out the obstacles "mountains" into the sea which Satan uses to block the rulership of God in our lives and others. His strategy is to steal from us God's provisions and to kill and destroy His works. We are to bind Satan and his work/workers and cast them out and into the sea - making Way for God's Kingdom !