You don't need a great QB to win a Super Bowl but you need a QB who can play great when it matters. Eli, Flacco, and Foles all had big moments during their run. We didn't really see that from Garoppolo (in fairness to him though, he wasn't really needed to do much in their two playoff wins to get there). He had his chance last night but just didn't have it.
He doesn't have a ton of experience though...just like Goff last year. Give a defense 2 weeks to prepare with an entire season of film and they can exploit an inexperienced QB. Even Mahomes, who has about the same amount of experience, did not look as great as he could be.