is there ever a good situation when karthus should go top? like, if there are certain champs top that would curb stomp most tops (ie yorick, teemo darius etc) could i send him in due to his ability to farm outside of their range?

Any top laner with a gap closer and/or sustain would beat him in lane. You would eventually run out of mana trying to bring their health down and when I saw your mana was low I would jump you. So no, he shouldn't go top.

He's too mana hungry to not only out-farm, by dodge and defend against most top laners. Basically, they could harass you much easier in most situations, unless you rush your slow, which would further perpetuate your mana dilemma if used excessively- this would be necessary against any competent opponent. Against a decent adversary, your farm would be null and you be at best sitting near your turret most of the time trying not to die because you're squishier than most top laners.

I mean you could throw him top and lee sin mid so lee sin can roam better, if you are blue side so he can still grab blues.

would he be a good pick against teemo, or yorick?

Maybe, and probably not.
If your enemy somehow has a passive farmer like nasus up top you could go karthus but... you can counter pick a lot better.