The story advances a recurring pro-Kremlin narrative painting Ukraine as a crumbling, artificial country, too weak and divided to make its own strategic choices and thus forced to accept external governance.
As a large and politically diverse country, independent Ukraine has elected six presidents since 1991, each with his own set of domestic and geopolitical priorities. Thus, the tenure of both Leonid Kravchuk (1991-1994) and his successor Leonid Kuchma (1994-2005) were periods of "multi-vector" balancing between Russia and the West; the pro-Western Viktor Yushchenko (2005-2010) was succeeded by pro-Russian Viktor Yanukovych (2010-2014) who, upon his flight in disgrace from Ukrainian politics (and from Ukraine), was replaced by Petro Poroshenko. The latter was elected on a firm pro-EU platform and anti-Kremlin rhetoric during a period of large-scale military aggression by Russia. Since May 2019 the 6th President of Ukraine is Volodymyr Zelensky.
There is nothing indicating that Ukraine's irregular foreign-policy record since 1991 constitutes an uninterrupted US effort to transform Ukraine into a "bulwark" against Russia. Consequently, there is nothing to support the claim that Ukraine is a client regime. See here for more cases on Ukraine being under external control.