Yes, it is generally flavorless, cheaper oil sold in large bottles.
In terms of cooking, it should be close to vegetable oil (if what you mean by that fits the description of 1). I think people use 植物油 (literally vegetable oil) and サラダ油 casually in the same sense/almost interchangeably.
Technically, vegetable oil is a collective term for edible oils from vegetables and サラダ油 is a vegetable oil refined in a certain way so as to meet some standard set by JAS. So comparing them may be a category mistake.
I suppose mustard oil is made from something similar to this plant. If this is the case, it is called [菜種]{なたね}油, which is used also as a general-purpose oil in Japan, like making tempura (deep-fried vegetables or seafood).
Added:
- So サラダ油 is a certain type of vegetable oil, including 菜種油, キャノーラ油, 大豆油 etc.
- Technically 菜種油 etc do not have to be サラダ油, but virtually it is the case that 菜種油 etc. on the market are サラダ油. That is, 菜種油 etc are サラダ油 that clarifies their ingredients.
- 植物油 means simply "vegetable oil", which does not necessarily meet the condition for サラダ油. This includes オリーブオイル(olive oil), ごま油(sesame oil) among others, which usually are NOT (considered as) サラダ油.