Both of Monaco vs Barcelona and Atlanta vs arsenal are not a huge surprise as many people are portraying it. Atlanta were the Europa league champions beating last season Bundesliga winner Leverkusen. Monaco too is a second team in french league last season. Both of Atlanta and Monaco were playing at home. Manchester City vs Inter Milan was much surprising for me. But its first round of league and there are plenty of matches to be played.
In my opinion, no result at the group stage can be considered unexpected - for top teams, this is the second (if not third) priority. If a serious team loses points in the group stage, this will not change anything - it will still get to the playoffs (even if through an additional playoff). But if a serious team loses points in the national championship, this can be critical for the entire season. Therefore, at the group stage, weak teams (for whom participation in the Champions League is not routine) have increased motivation and can beat stronger teams that lack motivation.