Large info dump warning, TLDR at end.
Personally I feel he is pretty good but takes more skill/knowledge than before. Generally if you save trading to when thunderlords decree is up viktor is very good at proccing it with an E, Q, auto combo which is fully available at lvl 2. Similarly at lvl 1 an Auto,Q, auto trade is good for procing thunderlords. If you try not to fight (unless you have advantage) except thunderlords is up you will find your early game trades are extremely mana effecient and you will get good damage without running oom. Similarly once you have the E upgrade, abuse that the double instances of damage from the lazor means just a Q or auto afterwards will proc thunderlords and give good dmg. As soon as you get off all your spells/proc thunderlords back off and wait for everything to be up again. A lot of vik's matchups come down to how well can you trade while dodging/minimizing incoming damage. Being excessively passive as viktor isn't as easy to do nowadays with the E nerf not letting many get by with just afk farming until they had their items.
Also I highly recommend trying different/situational builds since viktor's build path is extremely flexible to this day, and i didnt have as much success as i used to until I started building differently. Personally I get 15% scaling cdr from runes and in masteries I run Thunderlords as my keystone since he can proc it very efficiently and is a burst/combo mage. Generally I have then been rushing my first hexcore and get second dorans ring (this combined with hexcore solves all my mana problems usually). My next item depends on the enemy. If i feel i need it I get an early seekers arm guard (if you play well this and the seconds dorans ring combined with maybe exhaust or barrier will keep you safe from AD assasins, if needed get the full hourglass get it though). Otherwise I get abyssal if the enemy has a lot of magic dmg or AP assasins, the second hexcore upgrade, and sorc shoes. Then I get lichbane after second hexcore upgrade and defensive item if I got one. This makes me have a lot of burst for the midgame through lichbane and the sorc shoes magic pen, and lichbane/Qupgrade give me good sustained dmg/kiting capabilities. Once you have any more AP than the first hex upgrade and second dorans ring you will be back to oneshotting casters, and can often push the enemy into tower and roam if its good to do so. I will note that my earlyish lichbane, Q upgrade, and possible defensive item makes viktor play a lot like a TF or other kite mage and is nice so that he does more than wait for his full combo to hopefully oneshot someone, so it is a change of playstyle for some. Also a big thing for me is, dont get deathcap before voidstaff (unless your snowballing) cause the worst thing is while your busy saving up a million gold for deathcap the support gets locket and the tank gets a spirit visage and the fighter/assasin gets a maw of malmortius and suddenly you really want that voidstaff.
Does vik do less damage than he used to? yes, but his E upgrade dmg is still pretty absurd in base dmg and scaling so even if it leaves minions at 1 hp at lvl 7, it still is pretty effective against champions. The one thing about viktor, and it was a similar problem before the nerf, is that he is item dependent. This means if you relied on specifically using the oneshotting casters to farm safely and not interact with the enemy laner, then the nerf hurts a lot, and I recommend running scaling AP blue runes since with just a second dorans and the E upgrade you will be back to doing that at lvls 7, 9, and above and get your cdr from something else (lvl 8 is a bit funky). Since he is so item dependent, this also means you have to be pretty careful of ganks while going for these trades/all ins since viktor when behind does feel pretty bad until he farms back up. It's worth noting Viktor has never had the highest lvl 6 burst of all the mages, but if you use his ability to efficiently trade quickly before the final combo, you'll get pretty good success. Viktor is all about quick, and efficient trades before running away to wait for CDs. Once you get past the early game as vik, your on your way to the hyperscaling teamfight mage that viktor is a king of and feels just as good as before.
Really sorry for info dump, just thought id put in my two cents as a vik main.
TLDR:
-generally use auto,Q,auto or E,Q,auto to quickly trade and apply thunderlords and then wait to trade until thunderlords is back up
-Vik isn't able to simply be super passive due to E nerfs making it hard to oneshot caster minions without scaling AP blues for runes which hurts people who relied on that to deal with certain matchups
-Vik has super flexible buildpaths that can change based on playstyles and the enemy. try different builds until you find one you like and be more situational in builds
-Vik's matchups are somewhat experience reliant to know how much dmg the enemy can do, but generally try to trade when thunderlords is up or the enemy used a spell on minions and try to not get hit back (viks Q shield helps in this decently)
-Vik when behind is a bit of a bummer but he always was, just farm it out since you outscale many things and just one more item might make you strong enough to be relevant again
-Ganks hurt viktor a lot if they set you far back enough so be careful of em and ward and dont be too stingy with flash. Also his W is often good enough to stop most ganks but takes practice to find where to best place it, and is key to surving ganks as viktor.
Happy to answer any other questions and good luck! honestly everyone dropped him as soon as he got nerfed since his easiest play pattern (AFK farming in tough lanes with E) isn't as viable as before, and people drop things as soon as the smallest nerf hits it, not that this one isnt significant at all, it just isnt viktor breaking in my opinion.
) than the powerfarming power of E, which I will eventually get anyway.