I really don't think a game in and of itself can be pretentious. It's inanimate after all and has no thoughts and leanings of its own.
Gamers, game developers and game reviewers most certainly can be pretentious though. If I had to describe what makes a game pretentious it would have to involve the intent and attitudes of the people behind it. For example, a game that is independently developed is not pretentious in and of itself. However, if the person who made that game feels that they are entitled to some higher level of respect and credibility than other developers just because the big, nasty corporations had nothing to do with their game, regardless of the actual quality of what they've made, then that makes it pretentious. If a game satirises tropes from other games, that is not on its own pretentious. However, if the person who made the game's only intent was to point and laugh at the rest of the industry and say "Look at how drab and predictable everybody else's games have become!" without actually bringing any big new ideas to the table themselves, then that makes it pretentious.
So, my more succinct answer is that pretentiousness is defined less by the actual content of the game than it is by the attitudes of the people who make and play it.