Is natural that there is such response because in reality not all team in the UCL competition are really strong team, it is true that they are the best team from the domestic League that get tickets to play in this competition but some team in the domestic League that qualify are not necessarily able to play well, such as PSG for example and there are also several other team from less popular League.
Everyone will have their own way of assessing team and even the quality of competition, as long as the competing team are strong here I think it will still be quality and competitive match.
From now on, everyone can assess the abilities and strengths of each team that is still in the Champions League competition, especially when entering the playoff round and also the last 16 which will generally be much more competitive than the current round. And for the example of the team you said, it is true that not all teams can be good in the UCL competition even though the team is still very good in its own domestic league by occupying the top position and becoming a candidate for champion.
The UCL is the league to know the best in Europe but at times, it is not actually the yardstick to know the best, situation can happen, which is not limited to luck and lack of luck on the part of the less-deserving team and more deserving team respectively. Averagely overall, the UCL is a good league to know some of the best and of course, those reigning in a certain league might be messed up when another better league champion shows their supremacy. Some local leagues are generally weak but will not soon know until they play tourneys like the UCL since they are competing with the likes of weaker team locally.