He has scored plenty of times in big games though. I could be wrong but I believe the fixture in which he has scored the highest number of goals is the Milan derby.
Even in his poor CL scoring season, the goals he has scored were in big games but not in the smaller ones.
2019/2020 - scored 5 goals - 2 against Borussia, 1 against Barca, 2 against Slavia Prague
2020/2021 - only 1 goal - away against Real Madrid. Failed to scored against the weaker teams in the group.
2021/2022 - only 1 goal - away against Liverpool. Failed to score against the smaller teams (Sheriff, Shakhtar).
He's also scored in both SuperCup games, Coppa Italia final. In our Scudetto winning season, the most important match was probably Milan derby when we were close at the top of the table and he scored 2 goals that game.
Even last season, he failed to score against Viktoria Plzen but scored away Barca, QF vs. Benfica and SF vs. Milan. With Lautaro, he's never been the type of player to only score in small games vs. big games. He has his droughts but it has nothing to do with level of opponent.