Public figures may have a tougher time connecting with audiences when they become so famous and wealthy that they lose touch with everyday experiences, he added. An out-of-touch star is rarely likable, especially at a time when fans have grown accustomed to joining their favorite entertainers on social media as they document a date night or grocery run.

Rather than relying on big stars to pitch products, some companies are tapping TikTok tastemakers and Instagram nanoinfluencers to chase niche groups. “Brands are really looking for authenticity, for somebody that can talk about the brand in a way that others might not be able to,” said Adma Ortega, who handles celebrity and influencer relations for the Wieden & Kennedy ad agency in New York. “They want to talk to a specific audience, because you can’t talk to everyone anymore.”

Some companies break through the noise by ignoring likability altogether, as Samsung did in building ads around the reality TV star Christine Quinn, who once said of her role on “Selling Sunset”: “I love being the villain, and I wouldn’t have it any other way.”

Deciding on a spokesperson is increasingly complicated, involving metrics like social media follower counts, audience demographics, algorithmic calculations including the Klear Score, the Aspire authenticity score and E-Score polls. After the Super Bowl, Anheuser-Busch and other companies will study rankings like Ad Meter and the Super Clios to see how viewers reacted to their commercials.

Some firms still use the Q Score, a nearly 60-year-old measure of appeal that costs $1,750 per name. Clients get a detailed breakdown of sentiment among different demographics, and can also buy a full work-up of more than 1,200 personalities or a ranking of celebrities customized by their appeal to a particular audience. The score is used by ad agencies, movie studios, TV networks, lawyers and estates, said Steven Levitt, the president of the Q Scores Company.

The “Q” stands for “quotient” — a reference to how the rating is calculated, by dividing the percentage of respondents who say a celebrity is one of their favorite personalities by the percentage of those who have heard of the person. Another part of the rating evaluates unpopularity. The highest score ever, a 71 in 1985, went to Bill Cosby; nowadays, celebrities rarely score above the low 40s, Mr. Levitt said. Ms. Rudolph’s score is 19, three points above the average for actresses, he said.