He used to go mid or top. Now He fills the role of adc, but does not build as one. When going bot you will want to make sure you also have a tanky or cc heavy support, it helps alot.
Now morde can scale off ad or ap very well. E helps with harass in lane, while Q tends to have the higher dmg cap (all i can say is his Q scales higher off AD, but it's all about the 3rd Q).
Honestly he is Best suited for ADC now (at least the position)
*For top lane, i would run morde far more as a bruiser, go tanky and AD, focus Q.
-Only try to play this against melee tops-
*For Mid lane, don't pick into anyone with long range or heavy poke
(your slow and effective at close range)
However you are closer to dragon than top lane.
(Also, in top and mid, W is an underwhelming ability, good for when your jungler comes to gank though)