it's just not professional to act like that when working "live" with another person. Fair enough if she presented alone but not to put someone else into an awkward situation when trying to do their job. It's also disrespectful of the viewers.
IMO the best guests are those who don't take themselves seriously, are relaxed, show respect for the brand (however carp it is) and interact well with the presenter. A few come to mind - Rosa for YC, the Birkenstock lady, Helen McDermott the Van-Dal woman (she's a hoot) Simon Brown, Marie-Francoise the Kipling rep. I don't suppose they are 100% scrupulously honest in their claims but they are credible and entertaining.
Linda xx