Recommended Postgresql index type to be used with not equal () where condition
Indexes are good to reduce many rows to a few rows based on a condition in your query. Unless you have a very unequal distribution of values, a "not equals" condition will most probably not be considered for an index lookup.

So, it all depends on how selective this index would be. Unless it eliminates much more than 50% of rows (usually 90%+ is desirable) it's probably no better than a sequential scan.

See Use The Index, Luke