Set 288 for victory, Australia were rocked early on and could manage only 111 to lose the third test by 176 runs, following their defeat by an innings and 31 runs in the second match in the series.
Australia next host New Zealand and Cummins said the series loss meant questions were inevitable as South Africa took over from them as the number one side in the test world rankings.
“We lost two games in a row. I think it's fair to be asking questions and we'll be asking them internally as well,” Cummins told reporters. "We are a very good side but I think we need to get a little bit better if we want to get back to number one (in the world).
"Maybe that's a change of style, maybe it's a change of personnel in some instances. We'll work it out."
Cummins made the surprise decision to bowl first after winning the toss on the first morning, something that had been done only 11 times before in tests at Newlands since 1889.
As it proved, batting last was difficult on a wearing surface and Australia fell to the spin of Simon Harmer, who took career-best figures of 10-120 in the match.
"We knew it was going to be a high-scoring game,” Cummins said. “We probably thought it was going to do a little bit more on day one than it did.
"You don't win the toss and think you're going to bowl the opposition out for 100 or 200 but we thought we could get 10 wickets on day one and that would put us right ahead of the game."
Cummins said poor decisions cost his batters and that their chase was as good as over in the first half-hour of the day.
"Chasing 280-odd was never going to be easy but we all turned up today and saw a pretty clear path forward. I thought it was achievable but we never really gave ourselves a chance," he said.
"There were a few really poor decisions. I know it's a day-five wicket but I don't think it was drastic. There were runs to be scored out there; there were lots of balls to face.
"I think there are a few decisions the boys would like back. We have to get better. It's happened a few times now."
