Arda has been one of the best performers, or best, in most of the matches he's played at LW. He's had a good season in that position. Yes, he often played against weaker sides, but so did 10 other players.
From the stats thread:
01.Spanish Supercup - 1st leg
Sevilla 0 - 2 FC Barcelona
Goals: Suarez, Munir
Assists: Arda, Messi
YC: Suarez
02.Spanish Supercup - 2nd leg
FC Barcelona 3 - 0 Sevilla
Goals: Arda(2), Messi
Assists: Messi, Digne
YC: Umtiti
03.Lliga - Round 01
FC Barcelona 6 - 2 Real Betis
Goals: Arda, Messi(2), Suarez(3)
Assists: Alba, Roberto(2), Messi, Arda
YC: Suarez
04.Lliga - Round 02
Athletic Club 0 - 1 FC Barcelona
Goal: Rakitic
Assist: Arda
YC: Umtiti, Busquets, Suarez, Roberto
09.Lliga - Round 06
Sporting Gijon 0 - 5 FC Barcelona
Goals: Suarez, Rafinha, Neymar(2), Arda
Assists: Arda, Roberto(2), Denis
10.UCL - GS - Round 02
Borussia M. 1 - 2 FC Barcelona
Goals: Arda, Pique
Assist: Neymar
YC: Mascherano, Pique, Neymar
23.UCL - GS - Round 06
FC Barcelona 4 - 0 Borussia M.
Goals: Messi, Arda(3)
Assists: Arda, Vidal, Paco
26.Cdr - Round of 16
FC Barcelona 7 - 0 Hercules
Goals: Digne, Rakitic, Rafinha, Arda(3), Paco
Assists: Rafinha, Arda, Vidal
YC: Vidal
So aside from scoring vs Hercules, he has scored and assisted plenty of times against first division opponents, ending up as our third highest goal scorer and assist maker.
Furthermore, he has the second highest number of gold medals and third highest number of points, as voted in out motm threads. That's despite not being a starter.