Though actually, both of them seem to have pretty high tolerance levels; it's just that sometimes (often, in Roomie's case) they drink way, way too much. The real example of this trope is Lilian, who has a tendency to collapse on the spot not long after ingesting even the smallest amount of alcohol. Well... an even greater tendency than usual, that is.
Cerebus Syndrome: After the madcap opening act, and early Fish out of Water strips of Lillian adapting to Roomie's world, the love story becomes both more prominent and more muted, shunting Roomie's manic lifestyle to the side, especially after pressure from the advertisers reduced the frequency of nudity somewhat.
Crazy-Prepared: Roomie has left some clothes in peoples' places all over town, so no matter where she sleeps she already has something to wear when she wakes up. Not that she minds just going naked instead.
Does Not Like Shoes: Roomie doesn't seem to like them most of the time. Or pants. Or shirts. Or other sorts of clothing. But even when she's otherwise fully clothed she still doesn't like to wear shoes.
Lillian: (pointing down at Roomie's bare feet) Aren't you forgetting something?
Roomie: I am, aren't I? (comes back in the next panel wearing a headband) NOW I'm ready!
Double Entendre: Roomie needs approximately three seconds to come up with one.
Comes back as a Brick Joke when, at a later point, Lillian is given the option in a game of "Truth or Dare" to either tell the truth of whom her "first" was, or make up a double entendre on the spot, with a time limit on three seconds to make up her mind. She decides to go for the "think like Roomie" approach and delivers a combination of both just as the three second time limit expires.
Drunk on Milk: Roomie apparently gets herself drunk on water. Truth in Television, in that particular types of alcoholic beverages (red wine being a classic example) will leave alcohol in a person's liver; a sudden intake of liquid will thus flush this alcohol out, causing intoxication. Similar effects can occur in heavy drinkers.
Dude, She's Like, in a Coma!: Roomie briefly entertained the thought of thoroughly ravaging Lillian in her sleep, but settled for giving her a kiss on the cheek instead. D'aaawh.
Embarrassing Nickname: Lillians nicknamed "Lazy Tyke" by Roomie. It takes only moments for this to get twisted into "Lazy Dyke".
Even the Girls Want Her: Roomie seems to inspire this where-ever she goes. One character even explained that even though she is straight for the most part, she does make exceptions for shy little virgins who have never known a woman's touch... Roomie is an exception to that exception.
Everyone Calls Him Barkeep: No, it's not the actual barkeep (whose name is Jo), but Roomie herself. As explained in strip 3, it's a shortened for "Room-mate". He real name has not been revealed so far.
Face Palm: It's not like Matt has much of a choice about it.
Also a bit of a deconstruction, as Roomie's larger than life character is shown to leave her a bit lonely deep down, and guilty when people fall for her romantically as she doesn't feel able to return their affections the way they want. It also demonstrates why this trope would be attracted to hikikomori types like Lillian: Lillian, unlike nearly everybody else, does not put Roomie on any sort of pedestal, instead seeing her as a(n often rather irritating) human being.
Master of the Mixed Message: Roomie seems absolutely dumbfounded that some people may misunderstand her sexual antics as anything serious.
Red Oni, Blue Oni: Roomie and Lillian have the personalities pat, reflected in red and blue color schemes (to the extent color features at all), including "Roomie's" red-tinted real world in contrast to Lillian's blue-tinted dream world.
Suspiciously Specific Denial: Allan has definitely never had dreams about "steamy encounters with pink unicorns and dragons with rows of penis on its back."
Transsexual: Jak, at one time known as Julia, much to Roomie's shock.
Tsundere: Lillian, a Type A, who also mixes it in with a bit of Kuudere. Towards Roomie, obviously.
The Unreveal: When Roomie is asked (somewhat obliquely) in an interview whether she likes men, she purposefully misinterprets the question. When she wakes up having apparently had a threesome with two men, she objects of the straighter one, "he's... Allan!"
Visual Pun: It's revealed Lillian is still unconscious when it's showed that she's talking to talking screws, who tell her "you're screwed."