I don't think I'd say that Zayn isn't over. He's been consistently getting big pops, especially at PPVs. The 'Mania crowd ate it up when he and Owens started punching each other during the ladder match, and they easily got the crowd back into the show last weekend after Enzo's injury sucked the life out of the fans.
He's not as over as he was in NXT, sure, but he's definitely over. He just needs more time to grow on the casual fans who had no idea who he was prior to his call up. As long as he stays healthy, that shouldn't be an issue for him. He's one of the top five in-ring guys in the WWE. If they keep letting him put on awesome shows like the one with Owens last weekend, he'll easily become one of the most over guys in the company within the year.
As an aside, I've been reading that Owens has been involved in title matches at house shows, so I'd say it's definitely possible that he gets a program with Reigns after the Styles feud is over. Even if he doesn't win the title, that's some great exposure for him. I'd also say it's possible they give Owens the strap at Summerslam. Usually you don't see JBL and Cole agreeing that a guy is a future world champ on commentary unless the WWE has plans for them. Makes sense, too. He's arguably the best worker in the company right now and is one of the few guys on the roster capable of putting on a four star or better match with just about anyone.