Manchester United's performance in the group stage was really good? I don't think so. Although Manchester United won 5 out of 6 matches. But they have to fight hard to win every match. Even against comparatively weaker teams, they cannot dominate the match. If they can't dominate against weak teams, how can Manchester United win against big teams like Barcelona or Arsenal?
However, whether the Manchester United team is worthy of winning the title will be proved in their upcoming matches. Manchester United will undoubtedly become favorites to win the Europa League if they beat Barcelona.
I would say if you won 5 out of 6, no matter how you played, that should be considered as you played good and that's enough. I know it is not going to be that much convincing to just say that, but at the same time it doesn't need to be convincing, it just needs to be as good as possible for them to be there.
In the end, United will face Barcelona and they are a great team so they could of course end up losing to Barcelona, that's understandable, but that wouldn't mean that they were bad so far, they were amazing in europa and played very well. Sure some of the teams were weaker and it was harder than it suppose to, but they got the win in the end anyway.