GRC has brought more attention to rally in the USA and generated more interest than any other rally series has been able to. Yes, part of that is bringing in "celebrities" to drive. It's called marketing. It fosters interest for the sport even from non-car people who recognize the names.
If they were bringing in a bunch of Euros only who have been rallycrossing all their lives then Americans would've cared less and GRC wouldn't have lasted beyond a season.
I wouldn't say they have "zero car racing skills" or "of course they will fail". Mirra has gotten six podium finishes in Rally America including one 1st place and once won a bronze in X-Games beating drivers like L’Estage, a rally veteran who's been rallying his entire life. Bucky hasn't had much success in GRC yet but go read his racing bio and you'll see that he has a fairly extensive motorsports background.