Mitch4 send this in: “What is this ‘pour it on the floor’ in panel 1? Is that trying to be diner / bartending slang, and ultimately means something sensible like ‘no ice’? Or does he mean it literally, and if so why, and then how does it connect to the also rather mysterious words in panel 2?”
Comics I Don't Understand 
All I can think of is that it’s a bar at the beach, and pouring the drink out would attract the seagulls. Otherwise, I got nothin’.
A quick search revealed that some cultures pour out a small amount of liquor as a tribute/offer to the departed. I don’t see how that explains this one. Definitely a true CIDU for me.
Google tells me a mai tai is ‘a blend of 2 rums, coconut and spiced, with pineapple juice, orange juice, and grenadine.’ I thought that some ingredient might connect with pretzels, but no.
Unfortunately, the strip generates few comments on GoComics, so no help there.
From Gocomics: “‘UFO’ by Graham Harrop follows two duck brothers, Quagmire and Melrose, who were abducted from Earth and whisked away to Planet UFO. There, they run a two-star hotel for the alien administration.”
A quick perusal of recent strips suggests that the hotel’s amenities (e.g. the bar and restaurant) are outdoors in a bleak landscape. So it’s not a beach as such. Maybe the pig just likes mai tais? Who knows.
I’m baffled, too. Maybe an inside joke for regular readers, maybe a swing and a miss.
Without having read the comic I would interpret the joke to be about a misunderstanding.
One of the ways to serve a cocktail is with ice, or more precisely poured over ice. A common phrase to indicate this is “on the rocks”.
The surface of the planet is also made of rock.
The joke appears to be that the pig has heard the phrase “on the rocks” an took it to mean “on actual rocks”. With the surface/floor being made of rock he asks for the cocktail to be poured “on the floor”.
The barman perceives this as an insult. For why would he be asked to make a cocktail if it’s just going to be poured on the floor?
He therefore responds with a sardonic retort in which he asks the pig to duck. Because if he pours his drinks on the ground he might as well start throwing the food (pretzels) around too, that would be just as useless and wasteful. The pig will need to duck in this rhetorical situation if he doesn’t want to get hit by the pretzels.
The pig looks confused by this response from the barman, although this is hard to tell with the minimalist art style.
As a small aside, while “on the rocks” is used as a fancy colloquialism, it does have an interesting background. The rocks are likely a reference to actual cold rocks (usually pebbles from a creek) upon which drinks like whisky would traditionally be poured. However, ice is also technically a mineral and thus a rock as long as it remains in a solid state. So even though “on the rocks” is often perceived and used as a metaphor it’s technically a case of unintended metonymy.
I thought they meant ‘pour it on the floor so I can roll in the mud’. Doesn’t explain the pretzels, though.