Everyone fumbled, didn't they?

No. Right there in the last panel it says that they had nice rolls. DM just rolled even higher when he wasn't expecting to. Probably a nat 20 putting stealth just higher than all their spot checks.

Alternatively, they all rolled well for them, and the DM wasn't rolling for stealth (or the like) but rolling for what threats will appear.

Mild spoiler unless the next few strips deviate greatly from the source material, but I'm thinking it was a "roll twice more" or similar kind of result. ;)

I for one thought the GM was sarcastic, and that was the reason why Nami was putting the hands in her face. Because all of them did a bad roll

Same, I thought it was sarcasm with everyone rolling like a 3 and he was then Rolling Sanji's to save the day ... with a 1. Didn't think of Mr. 3/5 with Crit stealth though, or maybe that tiger dies from a crit fumble.

I'm thinking "everybody except the person with the skills to handle the problem sees _________"