Translation:My mom likes eating watermelons the most.
If this is supposed to convey that watermelon is her favorite food, then the English translation is somewhat stilted.
Imagine the following situation: a vendor gives you several options of fruits for you to buy to your mom. Provided that you only have a few bucks, you have to think about what your mom prefers. The "correct answer" (there are more than one in reality) fits here, because among the given options, your mom likes watermelon the most. But the sentence you provided doesn't fit here because it doesn't apply to a limited set of options but for all possible options in the world.
I put "My mother really likes to eat watermelons." and got "My mother likes to eat watermelon the best." as the suggested answer, which is bad English.
So is it the mother that likes watermelons more than anyone else, or is it that out of all the things that she likes to eat, watermelons come top? The answer has specifically come up with the comment that the definite article is needed. In the English that I speak as a native, it is unnecessary for either of the meanings.
Of all the items that the aforementioned mother enjoys consuming, she most highly prefers to injest melons of the Citrullus vulgaris variety. The Chinese sentence could not imply the other meaning you suggested.
"My mother likes to eat water melon the most" corrected to "My mother likes to eat watermelon the most". 90% of the mistakes I make are solely due to the English version of what is accepted and not mistakes in understanding the Chinese text.
Your answers are marked by a computer which compares your answer to a list of correct answers someone has to enter into the computer. It is simply not possible to anticipate all possible non-grammatical English language answers and add them to the list of "correct" answers. The list would be almost endless. You need to try to give grammatically correct answers as these are the only ones Duo will normally have entered. Duo is not criticising your English or trying to teach you English, they are just trying to match your answer with a list of correct answers to see if you have understood the Mandarin sentence.
That computer has numerous very peculiar English sentences it considers 'correct' and frequently marks grammatically correct English 'wrong'
In English, yes, there is a difference. 'Prefer' suggests that two things have been compared. 'I like this one better than this one - I prefer it'. Whereas 'like ... the most' puts one above many others. I don't know if Chinese has the same issue, but in English grammar (and it may not be the same in US English), when two things are compared a different adjective is used from when three or more items are compared. Hence 'He is the taller of the two brothers' whereas 'He is the tallest of the three/four/five etc brothers' I think that the difference you have highlighted is part of the distinction that is made in English between dealing with two items and several.
Watermelon should be singular because itbis the fruit in general she likes. She certainly never sits down and eats more than one whole watermelon ... well, my mom never would. I can't say about yours.