Clinton, who is in California for six fundraisers, expressed a touch of remorse for Bush, who is struggling in polls after a dismal debate performance.
"You know what, he's a very accomplished man and he is out there making his case," she said. "Running for president is really hard. Let me break that to you: it's really hard and you know some days are better than other days. I know that from personal experience."
Asked if she is quietly laughing at his recent campaign reset and slogan "Jeb Can Fix It," Clinton grew visibly amused.
"It's really hard to do this and people's campaigns change," she said with a smirk. "You start with something, that doesn't necessarily mean you'll stick with it all the way."
Kimmel remarked that his new slogan makes it sound like Bush "sounds like he's running a handyman business."
Clinton added, "If I were to advise him, I'd say, 'You know, there's a lot you can do about trying to fix things,' and maybe they should put a number on the side of the bus."
Kimmel had Clinton on for the first two segments of the show, longer than most guests, but politely ribbed Clinton for doing his show after sitting down with Stephen Colbert on CBS and Jimmy Fallon on NBC.
"I just want to thank you for doing us last," Kimmel joked.
The duo also spoke at length about Bill Clinton and the role he would play in the White House.
"Does the first lady typically pick out a new China pattern," Kimmel asked. "So would Bill do that?"
Clinton said her husband would play less of a first lady role and work with her more as an adviser.
"Really, I more imagine asking him what is the best way to create more jobs really quickly and get wages up," she said, taking her campaign line. "Because he did a really good job."
Clinton also confided that the former president would jump at the chance to run again.
"He would run again," Clinton said bluntly. "I don't want you to tell anybody that. If he could, he would."
But what about if they ran against each other in 2016? Clinton, after explaining why it takes a certain amount of confidence to ask people to vote for you, said, "If I were going to run against him, would I win? Yeah."
Asked about the Republican field, Clinton suggested that Republicans deny against climate change because of connections to oil and fossil fuel companies.
"I think some are doing to because they have strong supporters, people who may be from the fossil fuel industry, and they don't want to cross them so they adopt that position," Clinton said. "Whether they really believe it or it is political expedience, I can't tell."
And on Donald Trump, Clinton said that the Republican front-runner was once a supporter of hers but that all changed when he decided to run for president. Kimmel pressed Clinton about attending Trump's wedding, asking her what she got him as a gift.
"I went to his wedding," Clinton deadpanned.
Clinton also participated in a taped portion of the show. Kimmel convened a group of four children -- Sydney, Jayden, Belle and Andrew -- and spoke with them about having a woman president.
"They are too girly. They will make, like, girl rules," Jayden said, adding that they would give "free makeup in the world." Andrew added that they would decorate that White House and "make it all girly." "They might even paint it pink," he said.
The two girls, though, stuck up for a woman president.
"I think if there is a war, then she would probably make it stop so people could be more healthy and they won't die," said Sydney.
After a few minutes of honesty from the two boys, Kimmel invited someone "who could be president" in the room. Clinton walked in, and sat at the head of the table.
After talking with Clinton -- and suggesting free food, no school and free toys as issues the candidate should support -- Kimmel asked if either of the boys could support a woman president now.
No, they said.
"Well," Clinton said, "we need to have a woman to be president and then you would have more evidence to base your decision on."