I don't think Jinder has necessarily bad promos or delivery but I really wish he'd do more than just recite a script.
There is so much raw material you could use. He could talk about his climb from the bottom to the top. He could talk about how he grabbed the brass ring when nobody believed he could and that's why he's above everyone. He could talk about his body transformation. He could go deeper into why he thinks people hate seeing someone that "looks like him" as champion. There are so many different avenues to take but they keep regurgitating the same stuff week after week. I'm not putting the blame on Jinder but whoever is guiding him (cough… Vince… cough). Jinder has admitted that he's written promos before but Vince now is writing all his promos. I feel like Jinder Mahal is someone who's so appreciative of the opportunity he's been given, he's too cautious to speak up about any of his booking and just do what he's told.
Instead, you have a character who is an opportunist and even a bit of a coward but still acting like he's great. If they're going to go the coward route, that's fine, I'm not against it, but there needs to be signs that he's self aware of this. The context of his promos don't correlate with his actions.
My point is, this character needs to be a little deeper than what they're currently doing or he's going to become stale. We need to know who this character truly is.