Inserting Master database data into my database table
I very seriously question the use of a trigger here.  Your trigger is going to dramatically slow down the updates of your table.  That being said your query should look more like this:

INSERT INTO zzuomupdate(date_cu, Query_r) 
SELECT top(15) getdate(), dest.text AS [Query]
FROM master.sys.dm_exec_query_stats AS deqs
CROSS APPLY master.sys.dm_exec_sql_text(deqs.sql_handle) AS dest
ORDER BY deqs.last_execution_time DESC


The way you had it you were trying to pull 15 rows worth of information into a single column and that just doesn't work.  The INSERT INTO SELECT syntax is the way to go here.