Sweet flavor or mildness — describing something that tastes sweet or is mild/not spicy (opposite:辛口 'dry/spicy').
このカレーは甘口だから子どもでも食べやすい。
This curry is mild/sweet, so even children can eat it easily.
ワインは甘口と辛口、どちらがいいですか？
Do you prefer sweet or dry wine?
市販のソースは甘口と中辛がある。
Store-bought sauces come in sweet and medium-spicy varieties.
Flattery — praise or sweet talk meant to please or ingratiate.
彼は上司に甘口の言葉ばかり言っている。
He only tells flattering things to his boss.
甘口に乗せられて契約してしまった。
I was taken in by the flattery and signed the contract.
Leniency or naivety — used colloquially to criticize overly lenient or naive judgments/attitudes.
その審査は甘口すぎて基準が曖昧だ。
That review/inspection is too lenient and the standards are vague.