Poroshenko’s team was very active in promoting anti-Russian sanctions in Washington. It passed colossal Russia-related volumes of information on to the US Democratic party as part of these activities. The Democrats, however, mostly used this information against Trump rather than against Russia. Generally the information concerning Trump’s connections with Moscow was exchanged for US anti-Russian sanctions to please Ukrainian authorities.
For the whole period of the Mueller commission’s work, which is over two years, Trump could not do anything about it because media interpreted any his counteractions as an intention to hide his connections with Moscow. However, the official conclusion of the Mueller commission proves that Trump has not developed ties with Moscow. Hence it turned out that Clinton administration violated US laws based on the falsified documents passed by the Poroshenko team alleging illegal Trump ties with Russia.
This is a conspiracy theory aiming to diminish the Russian interference in the 2016 US presidential election by introducing unfounded claims about collusion between the US Democratic party and Petro Poroshenko.
Mueller's 448-page report though mentions Ukraine a dozen of times, mostly in a part which assesses Paul Manafort's ties to Russia and Ukraine, and does not contain a single mention of Petro Poroshenko.