This is bad practice to develop as a composer. I mean, for maybe really fast sections, but if you can't figure out the most simple passages in songs, MIDI files won't help you (won't help develop skill, they'll obviously help tell you the notes for the song).
You should practice doing things by ear. Hum the melody (yes, hum it) out loud really slowly. At the first note, keep humming that note until you click all the keys in the piano roll from C to the next C. When you find the matching note, move on to the next.
A great shortcut in this process is to learn intervals. I haven't had formal training in theory, so I'm going to leave someone else to answer how to do that. But basically if you know your intervals and you know what note you started at you can get a good idea on what the next one will be. Or, eventually down the line you can just recognize note patterns that composers like to use and put them down without going through each note.
@OP: It's not "poor taste", a lot of people do it, but doing things by ear helps you so much more as a composer.