Malzahar is pretty much Mid only due to his low mobility and high mana costs, Irelia is really only a good top, Nid can go to several different lanes.
Malzahar is a really strong champion, but the current Mid meta is high mobile champs like Fizz, Kat, and Talon. Plus the very early aggressive junglers like Lee SIn and Vi. He doesn't do so well against most of them, he is very susceptible to ganks and has low mobility so once you jump on him he has to flash or he dies. His ult does do a good amount of damage but without his W and Ignite, getting a kill with it is a bit hard. Not to mention how simple it is to cancel, ive had it cancelled and put on cooldown because a Pantheon would time his stun right when he saw my animation, my friend with Talon will jump to and silence Malz right when he sees the ult coming which also cancels it and puts it on cooldown. Truly his kit is very strong and he does tons of damage, but against the current popular Mids he falls off. In ranked he is very easily countered by a wide variety of very easy and popular champions.