Yes, it's possible to 'morph' between mouth shapes using Switch Layers. To do this, just open the switch layer's settings and enable Interpolate Sub-Layers under the Switch tab.
It can be tricky to setup though and you'll have to plan carefully for it. Basically, all the layers need to share the same 'base' drawing from which to create all your mouth shapes. Then, when you switch between drawings, it should smoothly morph between the shapes.
This approach sounds good in theory but personally, I find it can potentially limit what you can do with your character.
For one thing, every mouth shape absolutely must use the same base drawing, so you need to be sure that base is capable of being modified into any mouth shape you will ever want. If you add a shape that can't be created from the base drawing (usually an extreme,) it will not morph. This usually isn't a big problem if the transition is done quickly and you insert a frame or two of Switch Layer transforms between the two shapes, but it's something to be aware of.
Also, you can't use Groups as mouth shapes. I rely a lot on masking for mouth shapes so this pretty much kills the approach for me (most of the time.)
Another thing is that you can't tie this to a Smart Bone Dial. With Step it will just step the shapes as if Interpolate is disabled, and with Smooth or Linear it will just move through the shapes list linearly.
But you can use other methods for directly selecting the shapes for morphing (i.e, the previously mentioned Ctrl + Alt + Right Click, or Alt + C or D to cycle. If you use the latter, remember that Shift + Alt + Right Click will select the Switch Layer the mouth belongs to so you can immediately start cycling.)
The final reason I don't like using this method is purely aesthetic--to me, it tends to look very 'morphy' and digital. That's not necessarily a bad thing if it's the look you're going for. If the look bothers you, you can probably avoid the 'morphiness' of the technique by creating additional mouth shapes to make the transitions look more natural, but now you're talking about keyframing a lot more shapes so this is not necessarily streamlining the lipsync process.
Personally, I think a well choreographed set of still images can look more like it's 'hand animated'. It can even look surprisingly smooth when combined with basic transforms applied to the Mouth Switch Layer. And with a little practice, you should be able to knock decent looking lipsync animation very quickly this way. The only thing that will probably look better is to draw it all with FBF, but that's a ton more work of course.
None of this is meant to discourage you from experimenting with different techniques--just letting you know in advance where some of the pitfalls are with this one. There are many ways to create lip-sync animations in Moho and no one technique is the best for every situation or style of animation. With some characters, I find that a combination of techniques may work best, and sometimes a different technique for different head poses. Don't be afraid to try out several methods and see what works best for your character and project.
Oh, and don't forget to share your work! You might discover a new technique or trick nobody here has thought of yet.
