I know it's a bit subjective, but I find default-skinned TK apps to be much better looking than the average electron app.
[edit]
Here's a commercial application with a Tk gui that shows cross-platform screenshots:
I looked at the screenshots. That looks just like every piece of software made in the 90s before UI design became a thing people paid attention to.
I'm pretty sure this is making the opposite case you intended.
But the TK website not show it. Is very ugly!
This is a important point. Maybe TCL/TK is amazing but need better press to show of.
Is TK available to iOS/Android? How crazy a "TK React Native" could be?
I ask because I have a potential project where cross-platform is important and wish a good toolkit to use.
I will love to have a good Widget for multiplatform (and hopefully including iOS) but most toolkits have terrible looks. That alone is a turn-off.