This is the most reliable indoor block in Miami, which is exactly the problem with it on a wet afternoon.
From June to October, the storm builds through the late morning and breaks somewhere between two and four. It typically lasts thirty to forty minutes rather than the whole afternoon, and it empties every outdoor attraction in the city into the small number of indoor ones. This museum, the art museum next door and the covered retail blocks absorb most of that.
So invert the logic. Do not save this for the rain. Book it for a morning, and keep something genuinely flexible — a long lunch, the covered blocks in Brickell, a free Metromover loop — as the actual wet-weather fallback. That way a storm costs you nothing and the museum is quiet when you are in it.
If your rainy day is a Saturday in high season, arrive at opening or accept a later dome slot and a busy aquarium.
The saving grace: the Metromover station serving the museum park is free, elevated and sheltered, so arriving and leaving in the rain is genuinely dry. That is not true of most alternatives.