They said both Trump and Jae-in should be blamed for not specifying steps and deadlines for North Korea's nuclear disarmament, allowing it time to flawless its weapons program.
Sherman, a special advisor to former President Bill Clinton and policy coordinator on North Korea said the two leaders were acting more in the interests of their leadership standing than efforts to denuclearise North Korea.
At the signing ceremony, Trump said he expected to "meet many times" in the future with Kim.
Before a row of alternating US and North Korean flags, the leaders shook hands warmly at a Singapore island resort, creating an indelible image of two unorthodox leaders as they opened a conversation that could determine historic peace or raise the spectre of a growing nuclear threat.
If he was forced to negotiate by US pressure, it certainly wasn't obvious.
"The world will see a major change", he said.
He also said that before he took office previous year, "people were assuming that we were going to War with North Korea".
From the start of their meeting, Trump showered Kim with praise, calling him a "talented man" who "loves his country very much".
He said he was confident Pyongyang understood the need for verification that it was dismantling its nuclear programme.
New Malaysian Prime Minister Mahathir Mohamad, on a visit to Tokyo, said: "I hope that both parties will accept the fact that, in negotiation, both sides must be prepared to give in certain issues if they expect to reach a good conclusion". "At this juncture, I don't think we know enough to challenge or celebrate".
President Donald J. Trump of the United States of America and Chairman Kim Jong Un of the State Affairs Commission of the Democratic People's Republic of Korea (DPRK) held a first, historic summit in Singapore on June 12, 2018.
There is already much debate about who is the biggest victor out of the meeting.
The agreement signed Tuesday by Trump and Kim also included provisions for future negotiations, hinting at the possibility of more meetings between the USA and North Korean leaders.
"The vague language on denuclearization is not a breakthrough, it is a boilerplate reiteration of past statements", she said, adding: "It is far too early in the process for Trump to declare success".
But others say this is a clear victory for North Korea, which was in reality rewarded for its nuclear weapons program with the Trump meeting, elevating the regime to the top tables of diplomacy despite its behavior-including grotesque human rights abuses-sending a troubling message to other rogue states and groups.
Media captionTrump Kim summit: Win-win, or a Kim win?
"It's freakish but it might be that Kim Jong-un's interests and Donald Trump's interests align".
Other analysts and academics are underwhelmed by the agreement because of its lack of detail and specifics.
Lederman reported from Seoul, South Korea.
To probably even his own surprise, he got that and a whole lot more.
After pushing back too strongly on Trump administration demands it denuclearize immediately, which prompted Trump to temporarily call the summit off, the North immediately softened its tone to get Trump back on board.
He said it's meaningful in itself that the leaders of the United States and North Korea met, talked and signed an agreement that will carry more weight and significance than any pact previously made between the wartime foes. "Everyone walks away happy-Kim having met POTUS as an equal-having committed to nothing. Can".