만만하다 = "부담스럽거나 무서울 것이 없어 쉽게 다루거나 대할 만하다." (dictionary definition) = something/someone is easy to manage or deal with because there is no fear of consequences. "piece of cake". 만만하게 보이다 = something/someone looks easy. When used about a person, it usually carries a negative sense, like you don't have to worry about someone because they won't fight back no matter what you do to them - i.e. you can freely cross the line and hurt their feelings without fear of retribution. e.g. 너는 내가 만만하게 보이니? = Do I appear to you as someone easy to kick around?

which means , someone is being looked down on.