In the lore of ancient Greece, it was believed that anyone who looked upon the true form of a god or goddess would die because the sight would be too overwhelming to handle. That is one reason Athena disguises herself: she really likes Odysseus and Telemachus, so she does not want to kill them.
It is a common theme in these kinds of stories for gods and goddesses to disguise themselves as humans and animals. That way they are not seen to be directly interfering with human affairs. Often it is just a random animal or person, but Athena's disguise is not just any random mortal man: he is Mentor, a friend of Odysseus. This disguise allows her to get close to the two men without raising their suspicions because of course Odysseus' friend is going to be around and involved. It also guarantees that they will at the very least listen to her advice, and their respect for Mentor makes it more likely that they will follow it as well.
All in all, Athena adopts the guise of Mentor so as not to kill Odysseus and Telemachus while she does what she can to help them in their endeavors.