Is Kim Jong-un implicitly citicising Trump by praising the outgoing South Korean president?

According to the news the North Korean leader Kim Jong-un and the outgoing South Korean president Moon Jae-in exchanged friendly letters and Kim Jong-un praised:

the pains and effort taken by Moon Jae-in for the great cause of the nation until the last days of his term of office

Some articles in Western media like this made the hypothesis that with this praise he was preparing to blame his successor for the worsening relations. But actually the relations between the two countries are already worsening, it doesn't make sense to blame someone for things that happened before he was in office. Isn't there a more reasonable explanation?

During Moon Jae-in presidency North Korea held talks with South Korea and the USA shortly after these summits the relations began to worsen. So, if Kim Jong-un is not blaming the South Korea president for the failure of the talks isn't he implicitly criticising Trump?