Monster Girls - That's actually a legitimate question.

In light of the meaning of Metroid being "Ultimate Warrior" and Samus' genetic structure having the DNA of the last Metroid, I can answer this: She can turn into a ball. Without that suit, she can crawl all she wants.

This. Disregarding the morph ball for a bit, the varia suit looks pretty bulky. I don't think it'd be too easy to crawl in that; it'd be sort of like a knight in full plate mail trying to crawl. It's sort of doable, but not easy.