Index Maintenance on Clustered PK Index
I wouldn't worry so much about the fragmentation over 10% but over 90% fragmentation on a clustered index is a lot...but are you seeing any performance issues with that specific table when it has that much fragmentation? (If it's not a big table, it may not even be a concern.)
If you are experiencing performance issues with higher fragmentation then you can look into rebuilding (or oftentimes more recommended reorganizing because of its similar benefits while being less heavy of an operation) the index.
For a point of reference, if you had a table with only 10 rows in it, each row on its own data page, then you'd have 90% fragmentation  but the performance difference would be negligible compared to if you rebuilt it and it had 0% fragmentation. (This is a very basic example, and holds true for much larger tables to a point.)
By the way, I find this Brent Ozar Article by Kendra Little a good read on rebuilding vs reorganizing indexes.