This is a good question as it involves not just issues of language, (or cross translations thereof), but human thought processes and manifestation of such thoughts into actions or the taking of specific decisions resulting therefrom.
"没主见" & "没想法", what difference?
OP asked, "Is it correct to translate"没主见" or "没想法" as opinion, idea, or thoughts?"
Yes and no, which was why @Sanchuan said these are interchangeable, and he is right if we confine ourselves to just OP's Google translations which, being non-contextual, would be limited in its helpfulness.
Why yes and no?
First some well worn sayings about "thoughts" and "actions" which is what the whole discourse is about.
(1) Every action is preceded by thought. If we want to control our actions, we must control our thinking.
(2) A man of words and not of deeds is like a garden full of weeds.
(3) You'll never plough a field by turning it over in your mind.
(4) Think like a man of action and act like a man of thought.
Running through all these quotes is the consistent idea of the inextricable relationship / connection / fusion between "thought & action" So too is the bond and the differentiation between "没主见" & "没想法"
What is the "bond"? The bond is it all involves the germ of an "idea", a "germ of a thought" Whether it is "没主见" & "没想法" some thought processes are involved, (it has to be because "Every action is preceded by thought"), and hence the overlapping nature of definitions leading to their interchangeable usages.
However to suss out the differences we need to examine what these root words, 主 and 想 mean in their individual context.
主 has multiple meanings, but for our purposes, I think "primary" in the sense of having some defining core quality or value is what I would choose.
In 主见, where 见 here means a "certain view / outlook"
Thus 主见 would be, for our purposes, "to hold a primary point of view", i.e. an "opinion"
However, in order to have any kind of "opinion" formed, there has to be some preceding / initiating "thought process", i.e. 想法.
So, 主见 & 想法, what difference? The difference is 想法 is the invisible thought process, and 主见 is the observable manifestation, an outward expression of that thought process, whether in the form of a physical action or an uttered opinion.
We then have 没主见 & 没想法.
没主见 would be in the context of OP's question, someone who has not displayed, or failed to display, any kind of views or opinion worthy of serious or estimable consideration. Hence the negative connotation, in contradistinction to the "strong silent type" which display other positive qualities.
没想法 would be in the context of OP's question, someone who do not have much or at least noticeable thinking, analytical skills, thus having "no idea" in solving problems because 法 in our context means a "way" a "method" of doing things, and someone who is 没想法 is someone who lacks any form of sensible method, (in a sense "way-less"), in doing anything leading to a lack of thinking, analytical skills.
In conclusion, both "没主见" & "没想法" involves a commonality of initiating thought processes, but diverge in that one is a visible manifestation of the other.
Hence we get common, interchangeable English definitions like "opinion, idea or thoughts", because to have an opinion you need to first have an idea, a thought, and an opinion is nothing but an uttered thought.
When it comes to usages of the terms, well, context as always is king, thus giving us secondary meanings like, "indecisiveness", "thoughtlessness" etc.
