Trump is more credible than the FBI because he outlined his generally reasonable plan to address several of the serious issues facing the United States and is following through on a lot of his campaign promises.
Trump is focused on getting things done and the FBI, among others, are attempting to interfere or obstruct Trump's progress on much needed reforms in trade agreements, our tax system, over regulation, and corruption in Washington. Hell, they are trying to get him removed from office and don't even have a credible technicality to use yet.
Even many of Mueller's convictions are tainted. From FISA issues to plea agreements that may not have directly reflected the original charge, continued conflicts of interests, partisanship, and leaks for political purposes have destroyed FBI leadership credibility.
The FBI leadership has been politicized because in order for corruption to flourish in Wastington, protection, or even a supportive law enforcement is necessary.
You do recognize there is corruption in Washington, right? There is a sign the US public recognizes it through polls showing Congressional approval ratings below 20%.
The FBI's issues stem from a overriding political imperative and a lack of credible or substantive evidence against Trump. The FBI leadership wants to take trump down, but don't have the means to do it ethically or legally. The FBI leadership is betting high they will be able to find a technicality they can use as well as favorable Midterms to work with.
It is time for trump and Congess to clean house of FBI leadership and both the Left and the Right need to work for America's best interests over solely considering what seems best for their own parties.