She denied the comments were directed at Foreign Secretary Boris Johnson, who recently set out his own vision of a “glorious” future outside the bloc.
But in an interview with The Times she did go on to call for “serious people” to take charge of Brexit.
It comes as the Conservative Party gears up for its conference in Manchester, which has been preceded by cabinet divisions over EU withdrawal and is expected to be accompanied by mass protests against austerity and Brexit.
Ms Davidson said: “I think optimism, positivity is good.
“I think over-optimism and not recognising that there are practical realities that have to be faced, that have to be worked through and that complexity is not something you just skip over, that you actually have to work through, I think sells people short.”
Earlier this month, Mr Johnson set out a detailed vision for Britain's exit from the European Union that revived the claim Brexit could boost NHS coffers by £350m a week.
In an article for the Daily Telegraph, he insisted Brexit will allow the UK to “be the greatest country on earth” and “our destiny will be in our own hands”.
Questioned about Mr Johnson's message, Ms Davidson told The Times: “I don't take issue with optimism. I am quite often accused of having a kind-of, sort-of have-a-go element about myself as well.
“But there is the idea that this is quite hard and it needs serious people to do a lot of legwork and scan the detail to make sure we do get to a place where it will all be OK. It doesn't just happen by accident, it requires a lot of hard work and it requires application.”
She also said the £350m a week figure “wasn't a figure I recognised”.
Earlier this month, fears that Brexit could deliver a blow to the economy which the country does not recover from were raised by the Scottish Tory leader.
The remain campaigner conceded Brexit “is going to happen” but said she was concerned there would be a short-term economic hit and “we don't bounce back from it”.
Recent polling by YouGov put the Foreign Secretary as the top choice to take over as party leader from Theresa May, with support from 23 per cent of party members. He was followed by Ms Davidson on 19 per cent.
But Ms Davidson has given her backing to the Prime Minister's plans to stay in post after Theresa May said she planned to lead the party in a 2022 general election.
The Scottish Tory leader also backed the decision to hold the snap poll in June, claiming it stopped the SNP moving towards another referendum on Scottish independence.
“Theresa May might have lost her majority but by God she saved the Union,” she said.
Despite the backing of 19 per cent of activists in the poll, Ms Davidson has ruled out leaving Holyrood before the next Scottish Parliament elections in 2021, the newspaper said.
The Conservative Party conference takes place in Manchester from Sunday until Wednesday.
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