For a physical only document, the mark is something relatively difficult for others to perform. Even then, signatures by themselves usually only carry a certain amount of weight by themselves; either the risk of borne by the other party or additional measures are taken like requiring witnesses including notaries.

To answer the next question you’ll probably ask, it is considered to be far more trivial for someone to apply someone else’s mark digitally than it is in person. Anyone with a pdf editor can apply someone else’s signature to a document without their knowledge.