Given the preponderance of Frisk/Toriel art on this site, I believe this is an important question to accommodate those who wish to avoid incest.

The canonicity argument: In-game, their relationship is technically up to the player, which makes this complicated. However, Toriel does canonically place herself in the adoptive parent role at the end of the Ruins whether or not the player says they view her as such, and most art depicts the two living together.

The practical argument: If you're an incest-averse user and find yourself squicked (or not) by Frisk/Toriel, please provide your opinion on if you would prefer they be tagged as incest.

~*~It depends~*~

The incest/adoptive relationship lore tags are for the artist's intent. If that's the relationship the artist is drawing, the tag applies. Otherwise the tag doesn't. If intent isn't clear in the image, or you otherwise aren't sure, ask the artist.

No, that is because "artist canon" triumphs over "official canon".
I could make a comic where their stories changes entirely and that they are not related at all.