It's not that he's drinking alien milk that is wacky, but the presentation of it. Movies are a visual medium, so how things are visually presented matters, particularly so in Star Wars which has a big emphasis on how things look, e.g. Vader's suit, Snoke's hologram, etc. Most of the scenes with Luke put in a lot of inappropriate humor. It is the frequency in which the audience is expected to laugh that clashes with the tone, subject matter, and Luke's character. Robot Chicken uses this exact kind of dissonance to produce its humor, by putting in whacky stuff in its parodies of other works. Take something like the dramatic monologue in MacBeth and then give MacBeth diarrhea, an afro, and stick in a dancing cow in the background, and you will completely change the mood/tension/narrative of the scene.
Yes, Luke is a hermit, and it is perfectly sensible that he's drinking milk from some native wildlife. But you don't have to show it because it adds the wrong kind of emotions for the scene.
The main critique of the Luke scenes in TLJ is in how they presented Luke's character, both in its presentation (as addressed above) and the seemingly contradictory defeatist in old Luke versus the young Luke who turned Darth Vader back to the light side. From a narrative perspective, it is primarily an issue with the jump from RotJ to TFA, which produced a lot of problems, including the apparent collapse of the Empire in that time, the existence of Leia's Resistance paramilitary group (?), the First Order forming, and various other things. The audience simply has to accept too much in the time skip and a lot of it comes off as a hard sell.