You can say that about any politician who supports improving relations with any country.
If Nixon was a Chinese asset, how would his behavior look different? If Jefferson was a French asset, how would his behavior look different?
Or really anyone who supports policies that benefit someone else, like, perhaps the Civil War benefitted Mexico or someone by weakening the US with internal strife so then, what, should we conclude that the abolitionist movement was driven by Mexican assets? Should we assume that people who protested the Vietnam War were Soviet assets? Should we assume that people protesting the Iran War are Iranian assets? It's ridiculous.
The question isn't whether someone's actions are consistent with some hidden motives or conspiracy theory, but whether they're consistent with things that are already out in the open. It isn't really necessary to come up with these speculations to explain why Trump does things when his motivations are explainable through things that are factual and plain to see.
Also, if he were a Russian asset, he probably wouldn't have armed Ukraine in his first term.