As mentioned in @JoeW's answer, there are no states that Trump won over 75% of the vote share. However, there are some counties where Trump did win over 75% of the vote share (FWIW since county-level results can be misleading due to the varying sizes of counties). Nevertheless, they can still be an interesting reference.
The New York Times has a map that shows the "Size of lead" of Trump / Biden on a county-level.
As noted in the footnote:
Circle size is proportional to the amount each county’s leading candidate is ahead
On GitHub, someone made the effort to compile the county-level results for the presidential election.
2020 election results at the county-level are scraped from results published by from Fox News, Politico, and the New York Times.
These county-level election results are accessible at https://github.com/tonmcg/US_County_Level_Election_Results_08-20/blob/master/2020_US_County_Level_Presidential_Results.csv.
I downloaded the CSV file and ran the numbers.
- 31.2% of the counties in the US voted for Trump with >75% of the vote share.
- However, the average total votes of these counties is 9029, compared to the national average of 50,264.
These are the ten largest counties that voted for Trump with >75% of the vote share:
These are the ten counties with the highest vote share for Trump (don't read too much into this; note their total votes):