When your 1990 Olds struts need replacing do you also need a mtg kit?

You may not necessarily need them, but it would be aggravating to have to pull them apart again 6 months from now. If it were me, I would change the mounts and the springs while they are out, especially if you have more than 100,000 miles. If you hear a clunk while hitting bumps, or if the center metal piece of the mount comes loose easily after removal from the strut, the mount is probably finished- replace it.