You hit the nail on the head. Tcl/Tk doesn't get enough love, it's a really useful system even after all these years, it's gotten much better in terms of looks and it was always great in terms of what it could do. All Little does is make it more approachable for the 9x% of the world that prefers C, types, etc.
That's a bit of an unfair comparison. Gtk has lots of nice themes which AFAIK Tk can't use. That dialog doesn't exist on Firefox anymore, but this is what Thunderbird's preferences look like on my machine: https://my.mixtape.moe/fggaex.png
It's true, Tk can look okay on linux, but it will still be out of place when placed side by side with Gtk and Qt apps (Qt can use Gtk themes).
Yup, that's the price you pay for having it cross platform and avoiding C++. If I were making that choice today, might lean towards Qt, but I'm not sure. Tk's widgets are really pleasant to use, the last time I looked at Qt it was a lot lower level (may have gotten better, it's been a while).
When people say Tk looks bad point them here: http://www.little-lang.org/gui.html
Tk can look just fine.