[ineedasername]

No, lack of interest in a topic does not, by itself, cause awkward phrasing, run on sentences, poor structuring of the logical flow of sentences within a paragraph and paragraphs within the larger structure, repetitive language, repetitive language, repetitive language, outright incorrect word choice, overuse of the passive voice, ambiguous references... I could go on.

[fristechill]

Sustained interest in a topic solves the problems associated with how to express ideas about it. There's a co-evolution between developing an ability and having a motivation for doing so.

The flaws in writing you've mentioned are valid, but in the right hands they've all been used as rhetorical devices at some time or another.

[ineedasername]

Sustained interest does not solve issues of understanding how to properly structure paragraphs and larger portions of writing. It's not going to cause someone to follow Strunk & White's style suggestions when the person could not do so otherwise. I don't see how it could solve many other things I mentioned. I have first hand experience reading such work: 10% is outright bad, 20% merely poor, 50% okay but with some of the problems I mentioned, 15% good, 5% very good to excellent. (Rough approximations.) Further, writers in the okay -> excellent range can frequently perform at their same level on new topics or writing prompts. Interest is at best a minor factor.

Separately, the ability for skilled writers to employ problematic methids I mentioned deliberately and to good effect is not relevant to my original comment. I am not talking about excellent writers who can write fantastic work while subverting traditional writing style and the soft rules of grammar. I'm think that sort of writer would be correctly identified as human at least a little more often than others. I am specifically talking about the large number of people who can't do this.

Interest simply cannot overcome lack of basic knowledge on how write properly. No more than a strong interest in chemistry will overcome lack of engineering experience when building infrastructure for large scale chemical transport. Writing is a separate skill from knowledge on the topic about which the writer is writing. Chemical engineering is a separate skill from knowledge about chemistry itself. Strong interest can only take a person so far when it intersects with an endeavor that requires unrelated skills.

Interest may elevate someone's writing from okay or good to better but it cannot replace skills the writer doesn't have.