I'm level 88 now doing a non meta build and I'm wondering if I should already be doing higher than ~50 easily. I don't like running meta and I'm wondering if meta players were doing nm 100s at like level 70...

It's depend on the class and build. HotA, at levbel 90, can do T100 with proper gears, of course.

level 100 definitely

No , you are doing good

Basically what Ghosts said, it's more dependent on class/build/gear than level. Just keep trying higher and test it out. Sigils are pretty easy to come by so can keep testing.