To prove successful influence on elections, one would have to prove that the content of the ad influenced a voter's choice of candidate. This is hard, if not impossible to prove, by anyone, and Mueller certainly isn't the authority on why someone in Ohio or Pennsylvania or Florida voted the way they did. These 3 states sealed the deal for the election.
What is proved so far is attempted Russian influence on elections. This has been the status quo since the cold war. Had the question been asked in 2012 or 2008, the answer would have been yes as well. In 2017, the nature of spreading information to consumers has transformed dramatically thanks to the increased reach of social media, and the decision of the Russian government to use social media as an attack vector. This is par for the course in world geopolitics, as the United States government has engaged in principally the same thing for years across the world.
Also slightly off topic but - when a candidate says "Read my lips, no new taxes", or "If you like your doctor you can keep your doctor", and somebody votes for them, but instead they act against the interests of the voter, isn't the net result that the voter has been duped, regardless of by whom?
Regardless, I am curious about this particular anti-Russia wave though because it seems to be treated more seriously than other cases. For example when a sitting President says "Tell Vladimir this is my last election, after the election I'll have more flexibility", and also blasts the opposing candidate on the debate stage by joking that "the cold war ended 30 years ago" when he suggested Russia was a threat to American interests - is this not a good enough case for collusion with the Russian government to ensure that potentially objectionable geopolitical decisions are postponed till after a Presidential election so that a debate regarding their propriety can be absent from the political discourse during the campaign?
There is also the issue of large sums of money flowing to Washington lobbyists to influence members of both parties in the legislative branch, which is now being uncovered as part of the Mueller investigation - again this is not particularly new, as Paul Manafort, Roger Stone, Tony Podesta etc. made a lot of money doing exactly this for countries all over the world for decades.
The strongest case against the President would be if there was proof that Julian Assange's source for the DNC and Podesta emails was either the Russian government or someone working for the interests of the Russian government. These emails were very damaging to the Clinton campaign, but contained true information, nonetheless. Given that Podesta chose a trivial email password, it's also likely that some other entity had access to his emails. The challenge then would be to prove that the Trump campaign was a co-conspirator in this exercise, by perhaps coordinating the timing of certain releases with the appropriate campaign rhetoric, etc. What's also equally likely is that the campaign was clueless about the entire thing, and unwittingly profited from it. In that case, there would need to be evidence that if there are pro Russia decisions being taken by the executive branch or the military, and that these decisions are being made as a result of suggestions coming from the Kremlin. This, of course, would far exceed "influence on the elections", but "influence on the executive branch". Given that there is already Russian money flowing into Congress, this puts two thirds of the government in the pockets of Russia - i.e. unprecedented levels of corruption and probably some degree of treason as well.
For some unsolicited perspective - speaking as someone living in India - the geopolitical impact of an America that was supposedly free of Russian influence has not been a net positive for the world in my view. There have been a number of bad decisions and messes created in places like Afghanistan, Iraq, Syria, Libya and others. I'm somewhat confident that President Putin is not all that interested in whether Gorsuch or Garland takes Scalia's place, but is rather more interested in furthering his personal game of Risk that he plays on the world map. If the US government is now effectively a Russian puppet on the world stage, this is selfishly somewhat comforting since the Russians have been political allies with India for several decades. Putin violates the sovereignty of Russia's neighbours, as does China with India and others, as does the USA with countless interventions around the world, often seating a sympathetic figure as the head of state in other countries to facilitate this kind of thing in exchange for good deals. I find it difficult to accept that Hillary Clinton would have been a better outcome for the rest of the world given her decision making for the past 30 years of public life, and her track record as US Secretary of State. I don't know if we would be better off with two Putins as opposed to one Putin and one puppet Putin, if that turns out to be true in all of this. Major world powers like the USA, Russia and China are in a constant battle for territory and resources, which is something we could frankly all do without.