He's not a trash champion because of his win rate.
He's considered a trash champion because of his play style. The malzahar player has no real control over the champion.
You're forced into a particular play pattern and if it reaches it's potential power or not is almost completely dependent on the enemy's ability to play around or kill his voidlings.
This is what turned him into such a powerful support. You can't ignore the enemy malzahar, because his voidlings have too much potential power. Though if you're awake and deal with the voidlings, there is nothing the malzahar player can really do to kill you.
I think there's also a degree of anger towards his dumbing down. Realistically, none of the mechanics which were "added" to his kit, didn't exist before. They just weren't something that the layman wouldn't know about or utilize properly.
He always had potentially tons of damage on his voidlings if they were ignored, to the point that i'd sometimes pick up
on him (while playing AP), if i was behind and the enemy was focusing another member of my team. Even the little bit of attack damage would add up once you get 2-3 voidlings out towards the end of a fight.
The passive they gave him was basically just an automatic brain dead version of his voidling blocking which he could do before the update. I was known as the bane of blitzcrank kappa.
Then once people who knew how to voidling block started getting noticed after the rework, it became a problem, because you can't go around giving him two spell shields.
His ult was also rarely best used as a burst or assassination tool in a team fight, which people don't really seem to understand. The best way in the mid-late game to use his ult was to lock down an enemy who dove your carry or prevent an enemy from disengaging if your team was winning the fight. The damage was nice, but it any carry player worth their salt is going to have a
by the time they reach the teamfighting stage.
Basically, he's trash because his current design is just a regurgitated version of the old design, with less depth. (inb4 "There was no depth to old malz, you just W,E,R and get a kill", you know that narrative which didn't exist before riot announced his rework back in early 2016/late 2015)