моєму дядьку подобається салат з помідором but why wrong?
Your translation conveys exactly the same meaning as the correct answer, but it is not the exact translation of the English sentence. Duolingo has problems with creative translations, because there may be so many of them.
Agreed with you. There were a lot of "I like" or "you like" which translated as "мені подобається", "тобі подобається". Your translation should be accepted.
Why not " з помідорою"? In a previous sentence "with cutlet" was corrected to be " з котлетою". Both were additions to subject.