Why are mediocre actors praised and force fed to the public while truly talented actors are criminally underrated?

The mediocre actors fucked the producers or the investors.

Or, the mediocre actors are married to women who fucked producers for them.

Looks, OP...that's why. David Tennant is a Scottish actor who can play a wide range of roles. He has a niche on the small screen. Gerard Butler, who is a mediocre talent, has bigger name recognition and a Hollywood career of sorts.

Looks matter more than talent at times.

Talent rarely correlates with fame . Look at the popularity of the Kartrashians. Dan Brown novels sell like hotcakes despite being the literary equivalent of shit sandwiches. The entertainment industry is about making $$$, the casting couch, and nepotism, not artistic integrity. The 5% or so of the population that are genuine aesthetes are the only people who care about promoting real talent.

I reject your hypothesis