Does Deathfire touch seem weak but ok on damage over time champs?

It's not meant to deal A LOT, it's commonly used to deny healing (reduces healing/natural regeneration effectiveness) and can help secure kills, Cass and Jhin are popular choices for this

Over time, in lane, I feel like my poke does a lot more if I take dft. You lose a lot of damage in a combo to oneshot with, compared to TLD but DFT outscales.

I like DFT on Galio because someone has to stand in the Q to take TLD proc, but dft wont scale at all unless you build some AP. I build bruiser anyway since they nerfed his base damages.

A neat little interaction with Deathfire is that the damage is DOUBLED with certain tick timers. Deathfire ticks in 0.5 second intervals 8/4/2 times over the course of 4/2/1 second(s). while a lot of damage over time abilities/damage is done in 0.25 second intervals. Also Deathfire is applied immediately on all spell damage. This means that you're applying it at twice the rate you normally would, making it easier to compete with Thunderlord's, an obvious competitor. Just something to keep in mind when looking at its damage.