This could have been a wonderful RPG, set in the show's array of utterly fantastic locations, quests and side-quests sending you on various journeys, with pretty much the same combat mechanic. It could have been something that celebrated Adventure Time, made use of the amazing resources it had to hand. But instead this is an insipid, limp waste of it all, and a proper shame.
It remains a joy. It's calming, pleasurable, cute and tricky. It's Spelunky for people who don't like restarting all the time. But it's also its own distinct notion, with its focus on progression over difficulty. SteamWorld Dig is a really lovely, very fun time. What a great thing for a game to be.
On its own, out of nowhere, I'd likely be pleasantly surprised by a not-terrible game-of-the-movie, especially one aimed at kids. But in context, I can't believe you've played every single TT Lego game, and would far more strongly recommend you go fill in one of the gaps.