Can you Teleport a creature onto a Steep Hill? Yes. Does it require a Full Action? No. Teleport is an alternate method of transportation, so to speak, which abides by its own rules. Can you be Pushed onto a Steep Hill? Yes. For the same reasons, it is an alternate method of moving. So, now, can you Climb over a Wall onto a Steep Hill? Yes again, by the same logic. It is a different way of moving than the standard Move Action. Steep Hill's effect only affects normal Move Actions. It does not affect alternate ways of moving around.
A wall is still useful, as now not only does it require a Full Action, but it also requires the Climbing trait, prevents Pushes (if the Wall blocks Passage), and blocks Line of Sight which prevents many methods of Teleportation (though not all, as the Teleport ability itself doesn't require line of sight but many of the spells that grant it do).