Unfortunately, he likely is, at least least according to the latest reports about where Mueller is taking the investigation.
If you remember, Michael Flynn was fired by Trump, for lying about this discussions with the Russian Ambassador. Unfortunately, it appears Flynn had lied to the FBI at the same exact time, about those discussions, as Flynn has already plead guilty to that (lying to the FBI).
Mueller is looking into 1) whether Flynn's discussions violate the Logan Act and were ordered by Trump (long BS story but Mueller is looking for anything that might stick) or 2) whether Trump obstructed justice by asking Comey not to investigate Flynn (more BS but the problem is Flynn has now plead guilty).
I'm not saying that I agree in any way with Mueller's attempted cases against the President, I'm just letting you know that Flynn's guilty plea has given Mueller some options to try to entrap the President and possibly force his testimony. Those saying Mueller still isn't dangerous actually know little about what Mueller is actually up to.
Nobody has ever been prosecuted under the Logan act. And Logan is a dead letter after Obama before his election in 2008 negotiating with Iraq to oppose Bush, and by McCain negotiating all around the world at cross purposes to the USA. Last, every single president elect, BAR NONE has interacted and opened dialogue about the future with numerous nations.
As for firing Comey, Trump has an unquestioned right to direct the FBI, fire it’s director, tell him to end or begin an investigation, etc. It is impossible for his to “obstruct” one of his low level employees like an FBI director.
But in any case, this special counsel cannot file criminal charges against Trump. The most he can do is hope congress will take his report and try to impeach him. But Mueller cannot prosecute him. It can’t be done.