I also think Arsenal plays better than Barcelona or Manchester United. But it could be that they can't handle such a busy schedule playing well in England and the Europa League. It remains to be seen who they will play after the end of the qualifying matches. I think they'd be more comfortable playing with a weak club than getting into some other favorite like Roma, Barcelona or Monaco.
Roma and Monaco are not big fishes in Europa League. Big fishes are clubs which are built to play and win Champions League like Barcelona, Juventus, Manchester United. Roma and Monaco belong to Tier 2 clubs in Europe, not Tier 1.
Mourinho knows that his club will get more challenges from big fishes in Europa League than what they got in Conference League last season with that Roma was one of big fishes. Anyway he wants to help Roma to win Europa League by beating opponents one by one, round by round.
There is no doubt that Arsenal is among the favourites in the Europa League. They are lucky because Manchester United will face Barcelona, which means either of them will be eliminated and give the more possibility that Arsenal can progress to the top level of the Europa League.
But Arteta should be very careful and decisive because he might lose all if he decides to chase both Epl and Europa.
Winning Premier League is more important for Arsenal than Europa League. A Premier League title will help Arsenal a lot in next seasons. Their club will participate in top competitors in Premier League and confidence will be seen again in Arsenal after many years staying under Manchester City, Chelsea, Liverpool.
It can help Arsenal to successfully buy more star players in the summer. Winning Europa League is not too bad but its impacts on Arsenal will be less than winning Premier League. If you need an example, let's check Manchester United. After winning Europa League with Mourinho, they did not achieve any big thing so far.