I disagree. I liked Dack Rambo in Paper Dolls and his own Sword of Justice series and I thought Jack & Jenna made a good couple.
I've never seen him in anything but Dallas. I think the actor was alright; I don't think he was a bad actor. It was the way the character was written that limited his possible value to the show.
Jack was one of those people who thinks his problems all come from the place where he lives so he keeps moving from town to town. People like him are looking for a geographical cure for their problems, not realizing that they are the cause of their own problems, so of course they have trouble in every single different place they move to.
Jack and Jenna got along well for a while. I never got the idea Jack would have had the kind of maturity Ray had to be able or even willing to become a stepfather for Charlie. Jack had some immature notions besides his search for a geographical cure for his problems. He didn't seem to think he needed money to live, but we all do. Who would turn down 200 million dollars for a piece of paper that said he was part owner of a company that he had no say in how it was run? He didn't even work there (in season 10) and didn't seem to have any other full time job either, but he'd rather have the sentimental value of owning 10% of Ewing Oil than the money? Then he sells his $200 million dollar share of Ewing Oil to Jamie for a dollar. He acted like such a child!
Also whenever something would go wrong in his life, or become difficult, he'd pull a Gary and run away. He wasn't even responsible enough to tell anyone, even his sister, where he was going. He'd just disappear. So I really don't see that he was even close to being mature enough to get married and become a stepfather to a teenager.
When Dallas began, every character was written in a way that he or she had intrinsic value to the show. Every character was written in a way so as to be valuable to the show. Jack was not written in that way. He was one of those characters they added to the show much later on but just added to the show without any real vision as to what value he'd contribute to the show. In running a TV show, just like running any other business, without vision you perish.