Is it possible to search pieces by difficulty level?

Yes!

Go to the Catalog tab on Metronaut and tap on Filters (see screenshot below). You can use Search Filters to filter pieces by difficulty, by Accompaniment Type (Piano, Orchestra, etc.), by Instrument, Genre and more.

By default your instrument's filter is selected, so that you only see pieces that have been originally composed or have been arranged for your instrument. 

In the filters window choose Only Original Compositions to see pieces that are originally composed for your instrument.

See also: Finding Pieces on Metronaut