Mueller (a Republican) was given a broad mandate by Trump's own Deputy Attorney General Rosenstein (also a Republican) to investigate...
"any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump; and any matters that arose or may arise directly from the investigation; and any other matters within the scope of 28 C.F.R. § 600.4(a)."
Thus the mandate is not just about the election, its about to what extent the Russians had/have/will have their tentacles on the President, which would undermine his ability to do his job properly or be compromised when formulating U.S. foreign policy towards the Russians.
If Trump truly has nothing to hide then he would be wise to fully cooperate with the investigation so it can be wrapped up expeditiously.
Conversely, if he does have something to hide then he is in deep trouble and all of these recent anti-Mueller propaganda campaigns we've been seeing on right wing media (Fox/Talk Radio etc) are likely an attempt to discredit the results of the investigation because Trump knows what he has done.