I don't play top much at all, but last night I took Skarner there, and managed to roll all over Riven. Of course, one match doesn't really indicate much, but it seems like Skarner could do quite alright top.
theres not many, if any, people he counters/natureally beats that goes top normally. then you have the fact he's super easy to kite all he has to catch up to people are ms boosts, no gap closers, nothing. ganking for him is easy tho. once he hits 6 he can just pull someone with flash ult. and he can help gank mid.
i can see it not completely sucking if you play super passive and the enemy is someone who pushes. you can farm under tower somewhat? lol.
he's just not a great choice. i went against a skarner top once, cant remember who i used but he just played super passive. made no real moves or aggression, and i had a 50 CS lead on him cause i froze lane near my turret and he knew he couldnt get close. i got a huge lead just from farming and 1 kill on him. he still farmed up and got Gp10 items but if he had just jungled he woulda gottten much farther faster.