He clearly messed up Russian foreign relations, leaving aside everything else.
Just because someone was the victim of propaganda after their death doesn't mean they weren't pretty bad during their life.
How did he mess up Russian foreign relations? I assume you are probably citing the often used accusation that his Prusophila led him to make the foolish decision of making peace with Prussia. However that accusation is not true, Peter was no Prusophile, he had lived in Russia nearly his whole life and Holstein was in a manner closer to Sweden than to Prussia. Rather than Prusophilia fueling his decision to give Prussia peace it was politics. Upon his ascension Britain was already well on its way to winning the Seven Years' War, with an upsurge international dominance. Meanwhile Prussia was broken and weak. By making peace Peter repaired relations with the world power of Britain and also prevented the dismantlement of a weakened Prussia. Had he continued the war Prussia would have been destroyed and there would be no counterweight to Austria, which had conflicting interests to Russia in Germany, Poland, and the Balkans. By ending the war and saving Prussia he maintained a German counterweight. Additionally the good relations he established with Britain and Prussia meant that neither would interfere in his war against Denmark, which brings me to my second argument.
The second most common argument against Peter's foreign policy is his intended war against Denmark. Many people find fault with his planned war against Denmark based off foolish misconceptions. The simple fact of the matter however is that the war against Denmark was politically smart. Having secured the neutrality of the major players who could influence the war Peter effectively isolated Denmark and made it easy prey for Russia. It is most likely that Russia would have war its war against Denmark and secured Schleswig. Through the reconquering of Schleswig Russia can actually do what it tried to do in the time of Peter the Great, be an influential player in German politics. And considering that both Austria and Prussia were battered after the Seven Years' War this was the prime time for Russia to establish itself in Germany and expand its influence.
Also just a note, Catherine actually maintained Peter's alliance with Prussia for decades and did not truly realign Russia with Austria until the Greek Plan.