Why is it correct to omit 很 in 小男孩嫌小鱼儿丑?
When there's a verb before the object, you usually don't need 很 before the adjective
Since 嫌(dislike) is a verb, 嫌鱼儿丑 ( 這鱼儿我嫌丑) is a perfectly acceptable sentence,
More example:
天氣很冷
怕天氣冷 (天氣我怕冷)
這詩很好
讚這詩好 (這詩我讚好)