Metabro was a bit of a... well, we won't be rude now, will we, even if they are rude to us, eh? Anyways. In some Goomba vs Speedwagon thread this was brought up. This will be like my fourth calc of Goomba.
Basically, Goomba can survive this explosion. We don't know where in the castle they were, nor how many walls were between the Goomba and the explosion itself. But we'll try to give a good lowball on the feat. The explosion itself I'd estimate at High 8-C/8-B.
CalcLook at these images. Same POV, so pixel scaling should work. Additionally, in that second frame, we can see Bowser immediately after he is ejected out from the castle.
This is probably the best we'll get to a height for Bowser, standing at 2.33 meters. It'll do. EDIT: Imaginym has linked me to something (see comments) that says Bowser is 2.62 meters tall.
In that frame, Bowser is 12 pixels tall.
1 Pixel = 21.833 cm
Explosion = 586 Pixels = 127.941 meters
Halve for a radius of 63.971 meters.
R = Y^1/3 x 0.28, where R is radius in kilometers and Y is yield in kilotons of TNT. Plug in our radius to get a value of 11.84 Tons of TNT, or very much baseline City Block level.
Now, we will make two ends. In one, Goomba is near the epicenter of this blast (say, ten meters from it). This would mean the blast started in the throne room, and that the throne room was the middle room. Next, and less likely, we'll assume Goomba was towards the outer bits of the castle (180 Pixels wide = 39.30 meters). If we want to make no assumptions, this would be a safe lowball. As a high-ball, at the end I'll do from three meters, essentially meaning he was roughly on top of the source.
So, ten meter radius sphere for our high-end has an area of 1256.64 m^2. Our low-end sphere has 15349.85 m^2.
I = P/A, Power in Tons divide by Area.
High-End Intensity: 0.00942 Tons of TNT
Low-End Intensity: 0.000771 Tons of TNT
For the sake of ease, we'll assume CA of a Goomba is comparable to that of a grown human, since, although small, they are much wider. CA of a grown human is 0.68 m^2, multiply this by intensity to get durability. This would be effectively ignoring any barriers in the way of the blast and Goomba (and anyone else in the blast, too).
High-End: 0.00641 Tons of TNT, 2.682e7 Joules, Room level
Low-End: 0.000524 Tons of TNT, 2.192e6 Joules, Wall level
For a very high-end, we can say the Goomba was a mere 3 meters away. Surface area would be 113.1 m^2. Intensity would thus be 0.105 Tons of TNT. Multiply by presumed CA of our Goomba to get 0.0714 Tons of TNT durability for the Goomba, or 2.987e8 Joules, Room level. Taking this would mean Goomba would essentially be on top of the blast.
To my knowledge, the mid-end is most likely.
Goomba and Others Tank Blast (Low-End): 2.192e6 Joules, Wall level
Goomba and Others Tank Blast (Mid-End): 2.682e7 Joules, Room level
Goomba and Others Tank Blast (High-End): 2.987e8 Joules, Room level
Not sure if this was a special Goomba, considering he was alongside Bowser, so it may or may not scale to mundane ones. I just calc'd the feat, unsure of context. Edited with Imaginym's suggestions, the blast itself was upgraded to just above baseline 8-B, but durability to tank it was not upgraded by much.