It's not that they're less imaginative. In terms of creativity, anywhere is fair game. The real issue is that both Japan and America (Europe is pure and wonderful) have their own common design trends, for good and ill alike. But when someone usually talks about Westernizing a product, what they really mean is making it more palatable. Sanding off all those rough edges and making it accessible to the lowest common denominator. The removal of anything and everything that could potentially drive people off and the addition of superfluous elements dictated by marketing teams and corporate executives.
In short, Westernizing something means making it as bland and boring and safe as possible in order to appeal to a mass market, and in the process rendering it utterly worthless. You might realize that most of AsGryffynn's arguments essentially boiled down to this.
That's not to say the Nasuverse hasn't already been shaped by outside forces to some extent. FSN and Tsukihime had porn inserted so they could sell. In-jokes are replacing new content at a steady pace. And GO has slowly allowed the the 'verse's once careful world building to fall prey to many common design trends, tropes, and archetypes prevalent in Japanese video games, animation, and light novels. But even then, the Nasuverse has managed to retain its individuality mostly intact. Nasu still has creative control and exercises it appropriately, and a Type Moon story still feels like a Type Moon story no matter how you're experiencing it. In general the Japanese markets, or at least the kind of market that the Nasuverse occupies, tends to be more tolerant of those rough edges, likely owing to its sharper focus and smaller fanbase. That market allows Type Moon's indie roots to be preserved. That's partially why you so much more often see weird shit in anime compared to Western entertainment. Because buzzwords like "decency" and "tasteful" and "modesty" don't apply nearly as strongly. Because creators have realized that it's okay, and in my opinion better to appeal to a niche with a focused product than create something everyone and their mum can enjoy. That carries with it some bad, obviously, but it brings in far more good as well.
That's why I'll always oppose anyone who wants to change aspects of a work so it might better appeal to the masses.
Now someone that actually speaks moon can come in and call me a filthy weeb.