Your photograph really doesn't look all that bad. Mine come up wild in the garden like weeds every year so I never start any indoors.

This individual seedling doesn’t look too bad, but some of my tomatillo plants have almost all yellow leaves...

Yellow leaves that have burned borders can be indicative of chlorosis, which is nitrogen deficiency. Not really necessary to fertilize seedlings, but what I'm seeing in the picture looks a lot bigger than a seedling. It also looks kinda big for the pot that it is in.