Saying something is "a walk in the park" is saying that the person can do it as easily as taking a walk in a park, so you don't have to fear that you will mess it up. It is a reminder, often, that it is something that is within a person's abilities, so as to calm their potential fear.
It *is* a metaphor because it compares the task to something. If you put the word "like" in front of it, it doesn't change the meaning, and it becomes a simile.
This is not an exaggeration. An exaggeration plays up the high points in a situation. Hyperbole would be talking about the fish that got away being three or ten times the big one you caught. Hyperbole also happens when fish stories and war stories grow bigger and more vivid with each telling.
In contrast, this is downplaying the seriousness or fearfulness of the situation. It is an understatement.
In fact, hyperbole and understatement can both be metaphors, but one exaggerates, and the other downplays.