I'm a Japanese working in Tokyo for over 10 years. I
Let me answer to your question one by one.
>For travel to places like Hakone and Kusatsu onsen, will there be a strong WIFI signal?
As others on this board have already said, there are some free WiFi spots as both places are popular tourist spots.
However, there are 3 issues you should consider about.
1. There are SOME free WiFi spots, so you don't expect to be get connected through your travel (if you are considering use only free WiFi).
2. Mostly they are not secure and not recommended when you use your personal information (e-commerce, bookings).
3. On some free WiFi spots, you need to do bothering procedures to log in everytime their free times expired.
>Will a portable pocket WIFI or simcard be better for travel to these places?
That's why, I would like to say to that question, yes.
>Is it better to buy these in advance before going to Japan?
Still you can purchase them after arriving Japan, I recommend you to pre-order it through online. By doing that, they deliver it to your places (ex. airports, hotels).
Lastly, I just put a (almost) comprehensive list for data SIM cards and WiFi routers to find the best suit for your trip style as well as a website for comparison between SIM and WiFi.
Japanese SIM or WiFi: Which Fits You Best?https://prepaidsim.visitjapanplaces.com/lp/check
All prepaid SIM card in Japan: http://prepaidsim.visitjapanplaces.com/prepaidsim
Rental Portable Wifi Service in Japan: http://prepaidsim.visitjapanplaces.com/wifi
Hope these help you.