In a pair of tanks that weigh 87.5 kg.

That's not that bad right? 15 gallons of gas could be 43 kg, and the gasoline fuel tank has some weight too.

Although, after google search, I see that the Mirai weight is 4075 lb, and the Prius weighs around 3100 lb.

That ratio looks depressing, but what would equivalent batteries weigh? 500 kg? 1,000 kg?

A Tesla battery pack of comparable range is about 500kg.

Note that the mirai also needs the fuel cells, which adds another 70 kg, so the difference isn't as stark

Once we start that game, we can also add the weight of the Mirai's 1.6kWh NiMH battery and other esoterica.

The proof is in the pudding: the curb weight of a Mirai is more than that of the larger and more capable Model 3.

How important is weight in their designs though? They're cars, so they don't have to hold themselves up, and they probably both have regenerative braking

You are right, weight isn't extremely important, especially with regenerative braking. It is only interesting in the comparison with battery electric cars - if the hydrogen car is even heavier, then there is basically no advantage left for the hydrogen car.