Using broad brush strokes, here's your question currently.
- "I just started working on a large, legacy application."
- "There appears to be a lot of technical debt within the codebase."
- "I can't find any documentation within the code."
- "Is this normal and should I get over it?"
I'm being a bit glib with my paraphrasing, but that pretty much nails it.
Items 2 and 3 are answerable, and may actually already have duplicates on the main site.
And Robert Harvey has a pretty decent post providing some measure of evidence about technical debt and how often folk pay it down.
The 4th part, which is the one I think you care the most about, is a poor fit for stackexchange because it's purely opinion based.
And there's really no way to salvage that 4th part of the question and make it constructive. It's a conclusion to draw based upon evidence that is presented to you. To play things out, pick the answer of "yes, that's typical from my experience, get over it." Now compare that against "no, that's not typical based upon MY experience. Quit and get a new job!" Both answers are equally valid and the conclusion isn't something you'll find backed by evidence. The conclusion is purely opinion based.
I'm not sure there's anything that can be done to salvage the question for the portion you care about.
Props for asking and getting clarification behind why it was closed.
Have a look at this Whiteboard transcript. MichaelT reflects upon some of his experiences, and mine reflect what he's seen. Spoiler alert: yes, get over it.