They are not interchangeable.
"樣子" is a noun. We use "樣子" for appearance, some action you can see. For example, "他的樣子有點奇怪" means "His appearance is weird" or "His action(what he doing) is weird".
"看上去", in fact, we use "看起來" more often, is a verb. "天氣看起來不錯" means "The weather seems good".