Overall, I did really like this movie! How they portrayed Chinese culture was quite accurate and made me slightly nostalgic, as I am Chinese.
The message of the movie is about love, and moving on from loss. I think Fei Fei acted logically; four years isn't really a long time to mourn, and I think there are many people out there who have mourned their loved ones for longer. Maybe the "parent died, kid has issues now" trope is overused, but I think this movie used it well.
The animation is cute, enjoyable, and fit for a kid's movie. Like others have said, it shouldn't be comparable to a Disney or Pixar movie as the movie wasn't produced by a big company.
However, I gave this four stars because there's definitely some weak parts. There's a lot of points that aren't explained well, like: how did Fei Fei's mom die? When did Fei Fei's dad and Aunt Zhong establish a relationship, and how did Fei Fei not know? How did Fei Fei build a literal rocket in a few months?? How did the other half of the amulet get into Aunt Zhong's moon cake? When Fei Fei and Ching arrived back from the moon, why wasn't the dad worried and why didn't he ask them where they went?
Some details about the side characters are quite weak too: How did Jade and Benji fall in love this quickly, and how is it important enough that Benji would want to stay on the moon? I also think our knowledge of Gobi's backstory wasn't enough.
The music was okay, but I had to listen to them a few times to have them become catchy. On my first listen, some of the poppier songs were kind of cringy. However, the more emotional songs were quite easy to listen to.
I feel like the moon city design wasn't delved into enough; it was fun, colorful, and futuristic, but the movie never explained certain lore; like the chickens riding motorcycles, moon cakes, and giant frogs living there.
TL;DR: This movie was great, as it had a realistic message and fun animation; but had many unexplained details and some songs weren't that good.