1. It only covered his first 100 days in office.

2. The overall average for non-neutral coverage was 80% negative, 20% positive. For comparison, for the first 100 days, both Clinton and Bush were around 60% negative. While Obama was only 40% negative in his first 100 days, that flipped to ~ 60% negative in his second 100 days.

3. EVEN ON FOX, Trump's coverage was more negative than positive (52% / 48%)!

4. Only 6% of Trump coverage in that time was about Russia/2016 election.

5. Trump's approval rating after 100 days was 42%, compared to Obama (65%), W Bush (62%), Clinton (55%), and HW Bush (56%). Even just based on that, of course news coverage would be more negative for Trump.

6. His first 100 days included the disastrous travel ban roll-out (whether you supported the ban itself or not, the roll-out of it was a complete failure), a failed attempt at Obamacare repeal, his National Security Adviser being forced to resign, and generally a lot of chaos as someone without political experience was new in the job.

7. He attacked the non-Fox press, a lot. Of course they would take offense to that and "fight back" by covering it negatively (which Fox would not).

No doubt non-Fox outlets have somewhat of a left-leaning bias (and some more than others), but holding up that 90% number as a definitive counter to the claim of Fox's extreme Trump/right bias isn't a very well-supported argument.