Is telling someone "That looks like a child's drawing" an insult or an opinion to someone's drawings?

I saw this lady's drawing and I think it looked AMAZING! Some people said it is just okay. One said, "I've seen better drawing than this."
One said, "That looks like a kids' drawing! Lol"

"It looks like a kid drawing. It looks terrible. You're an adult you can do better."

Wow how rude and harsh. Why are people so rude about people's drawings? My dad said everyone is allowed to have an opinion and their allowed to hot like it. I think people are so mean and brutal.
Do they expect them to be professionals?