Gail addressed the performance aspect of it; fragmentation is very low on the "fix it" list as far as performance goes.

If you want to address the fragmentation, here's a bit of help.
so a table with no clustered index would be referred to as a heap table.
you can rebuild a heap table, which is the same as defragmenting an index with a command like this:
Alter table SchemaName.TableName rebuild

A better practice would be to examine each heap table,and add a clustered index to them. The general rule is every table has a clustered index, unless you have a good reason not to.
So add that to your to do list: examine your healp tables, and create a clustered index on them.
there might be edge cases, like a log  table, where an an index might slow it down, but even then , i would doubt you could find a good reason to justify no clustered index.

here's a decent article on heaps and fragmentation:
https://www.simple-talk.com/sql/database-administration/sql-server-heaps-and-their-fragmentation/