Answer:
Grendel mocks and criticizes whatever heroic ideals that "Beowulf" symbolizes.
Explanation:
The epic poem "Beowulf" tells the heroic deeds of the protagonist Beowulf and his battles against the monsters Grendel and his mother and also a dragon. Through these battles, we can see that Beowulf has been depicted as the ideal hero who braved obstacles and brought victory to his people.
"Grendel", on the other hand, was written from the perspective of the monster and his point of view. The author, John Gardner, gave an insight into the famous epic from the narrative point of view of the 'enemy' Grendel. And through this process, it seems that whatever "Beowulf" considered ideal and heroic, "Grendel" criticizes and makes a mockery out of it.
Thus, if "Beowulf" is about heroism and its ideals, then the very same theme/ ideals are criticized and made fun of in "Grendel".