Is having Estimated number of Rows > Actual Number of Rows an issue to worry about?
In the particular scenario presented:

No, the difference between estimated and actual number of rows is not important. There are two key pieces of information to support that statement:


The query has qualified for a trivial plan; and
The query has further qualified for simple parameterization


This is clear from the Seek Predicate text, where the literal values for ISBackEndSystemMigrated and ISDuplicate have been replaced with the parameter markers @1 and @2:



This means that the plan selected is the same one SQL Server would choose for all possible literal values. Trivial plan reduces compilation time to a minimum for simple queries; simple parameterization promotes execution plan reuse.

In addition, the apparent cardinality mismatch is due to SQL Server producing an estimate for the average case (over all possible literal values).

In other cases, of course it can matter a great deal that cardinality estimates are inaccurate. The trick is knowing when it is important or not. There is no general safe rule of thumb here, much of it comes down to experience. Query tuning is not all science; there is room for art too :)