Firstly, you don't need "for their life". Earn a living already sort of means that.
Has never been easy is past tense. A close Chinese equivalent would be 一直以來做 "cartoonist" 不容易賺錢.
The second sentence, "it is not easy" only speaks about the present. It doesn't say anything about the past. 現在做 "cartoonist" 不容易賺錢.