Mord Utl useless against quicksilver item?

Qss isnt in tft

this isn't the right boards but to answer your question, yes it is.

it's like getting sweeper to disable half teemo's kit, or abyssal mask to disable the other half of his kit.

This isn't a new thing. High-impact CC ults often get referred to as "QSS tax". Look at it this way:

1. If someone has to buy QSS to stop your ult, that's an item slot they can't use towards their actual build. It's a reactive item, and slows them down.
2. If you ult and they QSS, that robs them of their cleanse so your team's other CC can hit home and they'll still die. Sometimes the split-second disrupt is enough to throw an entire fight.
3. If they QSS anything else, you can hold your ult and then grab them once it's down. Obviously this means your team needs more hard engage CC like Leona ult, Warwick ult, or aforementioned Malz/Skarner.

Start thinking of Morde's ult as a disruption tool and pocket counter-gank, rather than engaging or winning fights with it. It's way more useful to grab the enemy jungler when a baron contest is happening, so your jungler gets a clear smite, than it is to try and 1v1 the ADC because 'lol what's kiting?'. And if their jungler bought QSS too, then your jungler is probably further ahead anyway.