Toriel could have asked Sans, she could have somehow convinced the human hunter herself Undyne to help her, Muffet, the nerdy science monster Alphys, but she figured that those choices would have been too much for Frisk, most especially Undyne. Her goal is to instill fear into Frisk if she wandered the dangerous underground full of monsters looking for her. She wants to get one of them to scare Frisk so much to where she would never leave Toriel's side.

So Toriel landed with choosing Temmie.
Temmie has been praised for her many accomplished feats throughout the underworld!
Temmie has ascended above all gods in each of their pantheons!
Temmie has stories of yore told throughout many campfires and songs sung up to this day (all lies).

Temmie is dog monster and is just a regular shopkeeper in the underground who's trying to raise money for college if I remember right (real world aspects mixed in with fictional things like monsters wanting to go to college is always good comedy to me). She's also known for saying "HOI!" as a greeting to others and having poor, broken english skills. There are other Temmies like her in her location with only one of those Timmies who can speak full accurate english.

Temmie is an odd and silly choice, but Toriel belives that she can at least put some sort of fear into Frisk.