Wrong wrong and wrong.
mordes carry ghost, and building pricilas on him makes sense so that is a 2nd ghost every 60 seconds.
He can out 1v1 just about every champion in game if build correctly, few can really beat a morde, and his recent nerfs had nothing to do with his ability to champion fight just lane, but there is no real lanning period in DOM.

Who needs a global / long range interupt when your standing on their closest cap point mowing them down.

At first I was like hmm morde sucks but I worked out a few different builds and he really can be very OP in dom, have to build him pure killer though with little to no tankiness and you can just crush their team. Also nothing says win, than grabbing that AD carry of theirs and getting two cap points with it before he goes poof.

Morde is a poor pick on DM. Why?

1. Lack of bunched up minions to build shield on.
2. Recent nerfs weakened him.
3. Ult->Carries not as potent as kills mean less and the ghost doesn't help in capturing a node.
4. Lack of mobility.
5. Lack of CC.

EDIT:

6. No laning phase means he can't get a gear+level advantage which he needs to steamroll 1v1,2,3,etc.

1.) he still builds max shield from siphone and mace on champions, and actually builds it at 30% max from level 1, got buffed.
2.) recent nerf only affected minion shielding. And his mace slightly weaker, but his mace is actually stronger in DOM cause it's more common to be single target for double damage.
3.) Ult on Carry huge means you run to nearest cap point and take it. Ghost doing the killing / fighting while you cap, or double team them and kill then cap.
4.) about 5 different mobility items, and ways to make him faster. Most that make sense on an offensive morde.
5.) CC how needs CC when your morde? Also he should be using several items that have slows, so you fight morde you going to move at 30% or less move speed till your dead or if you get lucky manage to get a team jump, but chances are you will be stupid turn to help die with next siphon.

Also morde is a carry killer. Always has been always will be. Something key on DOM.
Oh look its a jax with stormshield, and 3k HP. lets fight him, look we down to half health. No problem, ULT now he down to 20% life, and I'm back to 65% health, with another 15% coming in next 10 seconds. and he losing 15% health, hmm means he has 5% health I need to strip, and he doing just fine handling that with my thornmail. Look every time I use my abilities. Heck have had games where I have close to 75% life/spell vamp. and hit into 600 damage single target mace, giving me 400 health from mace and full shield from it. another 650 hp. So how do you beat something that hurts you when you hit, it constantly life/spell taps 1k worth of life and shield, and if he ults gets 30% of your max health.

Best hope you bring a few friends with some serious stunning power and CC. Wait no one plays those champions, cause they tend to not be tanky dps right? Not that it matters as quick silver will take care of most of it.

Is it better to go tanky for interruption and defense in Dominion or is it better to just go glass cannon?

Key to killing peeps in dominion = burst damage
Morde's biggest weakness = early - mid burst damage
Morde es #1 farmer
There is no reason to farm in dominion, he can push sure, but his lack of cc and mobility more than makes him a dead weight early unless he's purely defending, and then he isn't doing anything against long range pokes -.-

Ya I have to second that his nerfs only effected the laning phase, which Dom has none of to begin with.

Learn to build Offensive morde. Magic Pen runes, start with mace, grab parts of gunblade, open with crush gives full shield, and for most champions takes large chunk of life. Once you get your sorc boots and finish gunblade usually early on. you start to become very unkillable.

Gunblade gives you super slow, and often get a ghostblade, + ghost + pricillas = more mobility than ever needed.

To answer post above you, build him lifesteal / spell vamp cannon. with a thornmail.
Idea is to counter jax / few life regenning melees. most ap is burst and you live through that easily then can finish them up. The ones that give you trouble are the "I out lifesteal your damage via auto attack types" and thornmail takes care of that.

Also most people forget that his ult going to take 30% champion life, and well with a high AP build you can get it up to 40% from ult. When you steal 40% of a champions life for you, it makes 1v1 fights a bit unfair.

Eh, every time I've gone Morde in Dominion I end up stomping. He's one of the few champs what does well against Jax, because he does heavy damage without autoattacking while being tough to kill himself. Morde works better as a defender IMO, as he's prone to being kited when his Ghost is down.

His ult is a lot less powerful though. It gets you kills in 1v1 fights, but afterwards the ghost rarely ends up doing much without towers to beat on.

don't forget about reduced healing in dominion, for ult purposes