There are many posts preaching for the choir, but I wouldn't call it an echo chamber. It's more like a dead sound chamber where the ideas dies in agreement. It doesn't bounce off the walls or resonate. It's already there so no answer is required.
Lemmy would benefit from more users playing the devil's lawyer, but I think it's too small for anyone to use their main profile for that, and alt-accounts would quickly get blocked or banned.
Actual users with opposing views wouldn't be of much help. Politics isn't very nuanced these days. It's not red or blue, left or right or whatever. It's polarized into a new duality: Those that give a shit and those who are proud idiots. Lemmy is on the good side of this and will not benefit from being more accepting of idiots.