Yes the call for the Electoral College vote to be delayed until Russia claims are investigated are justified. If the Russians did interfere with the elections, then there may be a very real reason for Electors to vote for a candidate other than the winner in their states. Unfortunately, even if an investigation did find that Russia was involved in the election, it would be difficult to prove their intent or the amount of difference their influence made.
The electoral college vote would not occur unless U.S. intelligence was convinced that the Russians' intervention did not impact the outcome of the election. We must keep in mind that the United States has the most robust cyber capabilities in the world, even greater than the Russians. It is inconceivable that an intervention of the scale necessary to turn the election in favor of Trump could dupe not only the FBI but also the CIA, NSA, and various armed forces intelligence units.
The claims that Russians hacked the DNC emails and leaked them to influence the presidential election shouldn't delay the Electoral College casting their votes. The emails hacked and leaked have not been proven to be fake or otherwise tampered with. Any corruption or information that reflected poorly on the DNC and it's candidates may have influenced a portion of the votes but in the end the fact that no one has proven any of it to be lies makes it unlikely that any votes influenced by it would change. Therefore delaying the vote would serve no purpose.
Ultimately, the United States tries to interfere in elections around the world, all the time. In this case, Clinton is on a wild goose chase to blame the loss in the election on anyone but herself. Russia must be laughing at people in the United States who believe it. In the end, Trump is the President and there is nothing to do about it.