It doesn't seem to make sense that the tag doesn't have 'dreemurr' in it, because she's Asriel's mom and Asgore's wife. If I'm wrong and I don't know why it's like this, let me know.

I don't think it's really necessary, since "Toriel" is never referred to as such in-game, and officially she's divorced in the canon. Secondly, there is only one character named Toriel, and adding _dremurr would just add unneeded complexity.

I honestly try to avoid last names, but I prefer consistency over anything (e.g., zoe_trent and russell_ferguson OR zoe_(lps) and russell_(lps), but not zoe_(lps) and russell_ferguson). It kind of depends though; sometimes it ends up kind of a mix.

Single names are fine though as long as they are unlikely to be used for anything else.

Asgore is reffered to as "Asgore Dreemurr" or "King Dreemurr" all the time, but Toriel is always referred to as "Toriel," and even says, in game, "Don't call me Dreemurr."