Depends on what kind of tinsel you're dealing with.
For the clear iridescent plastic kind, there's not much that can be done.  The stuff is horrible to work with and fries if exposed to heat.  I've had some limited success in smoothing it out by carefully stretching it, but the process is time consuming and rarely worth the trouble given that the "evil" tinsel shows up on G3/G4, which aren't exactly hard to get in minty shape.

For G1's, they actually have metallic tinsel that responds decently to flat ironing.  Never tried it myself but others have had success with it in the past.