Are you sure 90c was not right? Most intel desktops have a tjmax of 85 or 100c. And i'm sure they designed this cpu to run hot.
edit - from my research it seems that the TJMax is 125c on the mini 9's. Also Intel has stated several times that the closer the temp comes to the TJMAX the more the margin of error is decreased. Anything under 50c should be disregarded and interpreted as just under 50c.
So unless your CPU hits 125c with coretemp, you should be fine. 125 is not the max temp for the CPU, it is the temp that the CPU will start to reduce either the multiplier or frequency. At an undisclosed higher temp the unit will automatically shut off to stop the unit from being damaged. I wouldn't worry to much about heat unless your are hitting the throttle limit.