No, that doesn't sound right. The ribbon system does not depend on having a certain tautness nor stiffness to the ribbon; it has to work with a variety of brands and ages of ribbon. The only exception here that I have noticed is when the advance mechanism does not reverse and the ribbon gets stretched extra-tight. Some have reported that a slack ribbon "bounces around" (sounds like what you have) and as a result it flops a loop while in the vibrator, so you might experiment with putting a tiny amount of drag on the supply spool.

Every time I have suffered that problem it has been either some error of threading route, or some bent or sticking part of the vibrator. It has often taken me a surprising amount of time observing things to figure it out. When a bent part has been the problem, it seems to be quite fussy getting it fixed. There is not much room to work on the vibrator and the metal is springy so it has to be over-bent and spring back to where it should be. And it can be tricky to remove it and replace it (and I haven't had to actually remove one yet).

Anyway, careful observation of the motions is the key. Be sure all the operating linkage is clean.

The vast majority of these problems are caused by an incorrect ribbon path. In fact, I'd estimate that close to 50 percent of the typewriters I buy come with the ribbon incorrectly installed.

 Good news. And a cheap fix!