"I do not like lemon."
Translation:मुझे नींबू नहीं पसंद है।
I think of like it's saying 'lemon is not pleasing to me' (or with चाहिए it could be 'lemon is not desirable to/for me'. Then lemon is effectively the subject. So you wouldn't say 'lemon am not pleasing to me'. Same as in Spanish with 'me gusta' and 'me gustan' etc.. I'm not sure if that helps, or if it's quite the correct grammatical view to take, but I find it works for me so far in getting the right parts of verbs and cases etc.
हूँ is wrong here because it does not make any sense. है is more suitable. In hindi the words you use should be suitable to the sentence. So here, "I do not like lemon" will be. "मुजे नीम्बु नहीं पसंद हैं।" "मुज नीम्बु नहि पसंद हूँ।" Is not apt. हूँ is used when u are referring your self. In the given sentence you are talking about your choice that u don't like lemon. The main focus is on choice. Example of हूँ : "मैं भारत से हूँ।" (It means - I am from India) here the usage of हूँ is apt.