Shatner is a decent actor. There's an interesting clip here where he discusses his method:
By the 1980s theatrical acting was no longer appreciated, and Shatner's reputation had taken a hit because of how pushy he had (necessarily?) been during his lean years. By the 1990s, he took a cue from George Foreman and Leslie Nielson and realized he could reinvent himself. He gave the public what it wanted -- perhaps at the expense of some good performances.