Last season in the Europa League was a real example when they managed to beat the strongest candidate to become champions. Almost everyone was not on their side, but in the match they were actually able to become winners, including beating a club that was unbeaten in all competitions last season.
On the other hand, the difference is that because the Champions League is different from the Europa League, here they will get more pressure, so they must be ready for that by improving their mentality.
So far Atalanta has not started the season well, especially in Serie A. Atalanta was only able to win one of the first three matches and only got 3 points. Losing two consecutive matches has left Atalanta stuck in 12th place in the Serie A standings, so I don't think Atalanta will impress in the Champions League if they don't improve.
I think any team that tries to underestimate Atalanta will be disappointed because they are full of surprises, Atalanta is the current champions of Europa League last season after defeating the team that was seen as the favorite to win the competition and this season Atalanta is among the SeriaA teams playing in the champions league. They may not have started very well in their domestic league but you can not compare their performance in the champions League competition and that of their league because they wouldn't want to disappoint their fans by not performing well in the group stages.
Atalanta may not be seen as the favourite in the champions league competition but it takes a good performance for the chances of the team to win the league to be increased so if Atalanta shows good performance in the league it won't take time for their chances of winning the champions League be increased.