The supporting cast is where the jokes actually sit
John Michael Higgins and Cheryl Hines do the structural work that lets the two leads play the romantic line.

The architecture of the genre
Studio romantic comedy has a specific problem: the two leads have to be credible love interests, which means they cannot be ridiculous all the time. The comedy has to live somewhere, and the genre's solution, refined over decades, is to park it in a supporting frame. Think of Nora Ephron's pictures, or the ensemble construction Nancy Meyers favoured — in both cases the central couple is surrounded by people whose job is to carry the laugh lines the leads cannot afford to throw away on themselves. The Ugly Truth follows that construction precisely, and the two people doing that work are John Michael Higgins and Cheryl Hines.
Higgins plays Larry, the morning-show co-host, and Hines plays Georgia, his wife. The couple exist in the film primarily as a running demonstration of everything the film's thesis claims about men, women, and the gap between romantic performance and domestic reality. They are the joke made flesh: a marriage that has calcified into a managed détente, played by two actors who know exactly how to make that funny without making it mean. The pairing is not incidental. It is the comic engine the film runs on when the leads — Katherine Heigl's Abby and Gerard Butler's Mike — are required to play something more vulnerable.
Higgins and Hines, specifically
John Michael Higgins came to the role with a long record in ensemble comedy — the Best in Show and A Mighty Wind work for Christopher Guest, later the Pitch Perfect franchise — and he brings to Larry a particular skill: the ability to be oblivious in a way that reads as cheerful rather than cruel. Stuart is not a villain. He is a man who has made a kind of peace with his own cluelessness, and Higgins plays that with precision. The character's function is partly structural: he is the authority figure whose blandness licenses everyone else's frankness.
Cheryl Hines, best known at the time for her long run as Larry David's wife on Curb Your Enthusiasm, brings the opposite quality. Georgia is not oblivious — she is entirely aware and has made a different kind of accommodation. The comedy in Hines's performance comes from composure, from a character who has absorbed every absurdity around her and decided not to react. That contrast — Higgins's cheerful vacancy against Hines's controlled knowingness — is what makes their scenes work as a unit rather than as two separate supporting turns.
Together they demonstrate the genre's standard supporting-frame construction ↗: the married or coupled satellite pair who embody a version of the central relationship at a further stage, usually a cautionary one. The audience reads them as a possible future for Abby and Mike and responds accordingly — which is why the jokes land with a register the leads' scenes cannot quite match.
Why this matters to the film's economics
Supporting comedy of this quality is not free, and it is not accidental. Nicole Eastman's original script, then Karen McCullah and Kirsten Smith's rewrite, built these roles to carry weight — a three-writer credit on a studio project is often a signal ↗ that the structure went through significant revision, and one of the things structural revision tends to sharpen is the comic relief architecture. The supporting couple exists because the script needed them to exist, and they work because the casting matched the writing.

For a mid-budget picture running on a production cost of around $38 million (per Box Office Mojo's reported production budget), the decision to spend on Higgins and Hines rather than spread the budget toward more recognisable names is a considered one. Romantic comedy audiences in 2009 were sophisticated enough to know the form, which means they were also sophisticated enough to notice when the supporting frame was thin. Higgins and Hines are not thin. They hold the film's comedic centre of gravity in every scene they share, and they hand it back to the leads at the right moment — which is precisely the job, and precisely why that job requires actors who can do it.