Hi Travis, I would say that "我昨天吃面“is the "neutral way" just to say, "I ate noodles yesterday."
You *can* add 了, but it will change, perhaps not the meaning, but the "context" of the sentence. When you say “我昨天吃面了”, there is an implication that you've been wanting to eat noodles for a long time, and you finally got the chance to eat noodles yesterday. Or if today someone asks you whether you would like noodles for lunch, by saying “我昨天吃面了”, you are implying that you have already eaten noodles just yesterday and you'd prefer to eat something else today.
In other words, use "我昨天吃面" to state a simple fact that you ate noodles yesterday. Use “我昨天吃面了” to give an extra implication that goes beyond its neutral, past tense meaning.