Can you add index to let element; let i = index;" as you’d do with *ngFor?
<mat-row *matRowDef="let row; columns: displayedColumns; let i = index"></mat-row>
Or like so:
<ng-container matColumnDef="index">
<mat-header-cell *matHeaderCellDef> Index </mat-header-cell>
<mat-cell *matCellDef="let element; let i = index;">{{i}}</mat-cell>
</ng-container>
Working Example: https://stackblitz.com/edit/angular-acdxje?file=app/table-basic-example.html