Considering only Rock, Cena, Punk, Bryan, Jericho and arguably Ziggler are better
And Barrett, Sandow, Rhodes, Kane, Big Show, Henry, Riley, even Sheamus, and not to mention managers like Colter and Heyman. I'm sure there are more on the roster who haven't been allowed to show their potential/I've forgotten about.
Del Rio's terrible on the mic, and the promo on SD, while maybe his best, was still only decent. I don't know how an all-Spanish promo would sound from him, and it wouldn't really work in WWE, but due to his obvious limitations in English, it's really effecting his promos and making them damn near impossible to listen to.
The only two things he has going for him are one, the fact he draws in a demographic that WWE doesn't really have anyone else to reach to that's better since Mysterio's almost done himself, and two, his wrestling ability. But due to the first thing I mentioned, he's been getting pushed hard, which is fair enough, and tbh I'd much prefer he get pushed hard on Smackdown than on Raw.