Cera smiling at them before they start laughing (which was never intended to be mean-spirited due to their reactions when she leaves) is a sign of relief, yeah. I thought that was pretty obvious. Even as a kid, I always viewed it as being her relieved that it was just them.
Cera crying afterwards, I never thought that was because of them laughing at her. I always thought it was because she was feeling guilty about leading them the wrong way and the danger she put herself and the others through, all because she wouldn't listen earlier.
The tar monster never seemed gross to me. Or scary for that matter. To me, it was just a bunch of tar and twigs and dinosaurs stuck together. Even as a little kid, it never bothered me in the least. It's not my favorite scene of LBT, but it's certainly not my least favorite either.