When arriving on Sakaar Thor is subjugated by a simple electric net.
Since he is the "God of Thunder", wouldn't he be immune to that? Or was it not a common electric energy?
The movie doesn't address it explicitly but we can guess what happened.
This question is about a plot point of a recently released movie, so obviously spoilers ahead
In short: Thor is subjugated by a simple electric net because, at that time, he is not the God of Thunder. He truly becomes it during the movie.
First of all, Thor produces thunder a few times in the movie:
The scene described by the OP appear between events 1 and 2 above. Clearly, at the time of event 2, Thor has lost his "mojo" and fighting spirit: the 9 kingdoms are in chaos; his dad just revealed he had a hidden, evil sister and died; the said sister came back from exile, overpowered him and Loki, and destroyed Mjolnir; and he is lost in space while his people are in danger.
At that point, it feels that "God of Thunder" is more a title for Mjolnir's wielder than a real set of powers. Thor has to believe it to be the true God of Thunder. This is made clear by the vision of Odin (just before event 4 above): Odin explains that Mjolnir was just a tool for Thor to focus and harness his power. Thor was wrong believing that he lost most of his powers by losing Mjolnir ("Are you the God of Hammers?" asked Odin).
Finally, the movie is about Thor's journey from Asgard's protector ("Are you a warrior?" people on Sakaar keep asking) to Odin's successor. At the beginning of the movie, Thor was not interested in ruling Asgard, but decided to investigate the Infinity Gems's threat. His lost eye clearly symbolizes that he became similar to Odin by the end of the movie. Even better than his father: if one must not look for war but should be ready for it, the best ruler is probably someone that was not looking for power.