Please, don't let this game degenerate to the level games like WOW and others are at where players are jerks and park their mounts on top of NPCs while they talk to them.
At least in this game E interacts with npcs and F for players, so even if there's a pack sitting on top of the npc, aim your mouse in the direction and hit E and it will generally pick it up.
I've noticed this being inconsistent. Sometimes I am forced to dismount, other times not. EIther way, as Seraseth said, since the interaction keys are different I haven't had any issues not being able to click the NPC.
You're pushed back a bit from npc's when you talk to them, whether mounted or not, aren't you? I haven't seen people on top of npc's, rather, arranged in a neat semicircle around them (which obviously they didn't do themselves).