When using Tar Shot, you should get 2 explanation screens, the first informing you that the targets speed has fallen, and the second that the target has been made weak to fire. In a normal battle both messages appear, and if you check the status of the targeted Pokemon you will see Tar Shot: This Pokemon has been made weaker to Fire type moves.
Against a Tera Raid Pokemon However, only the speed drop message plays and if you check their status, no Tar Shot effect has been applied.
So No. Tar Shot will only drop the speed of a Tera Raid target, but will not apply a weakness to fire.
Hope this Helps.