JR doesn't care. Immigration would care if you did paid work for a Japanese employer or enrolled in a school, but JR just asks to see your passport. As long as your passport is stamped with "Temporary Visitor," you can use a JR Pass. They won't ask what you're doing during your temporary stay, and you don't need to tell them.

(I'm assuming that you know that you need to buy the voucher outside Japan.)

However, unless you're going to be traveling long distances (at least a round trip between Tokyo and the Kansai (Kyoto-Osaka-Kobe) region or making a LOT of side trips, a JR Pass may not be such a good deal, since you can't use it on the Tokyo subways or the regional private railroads.

For transportation on the subways, buses, and private railroads in the Tokyo area, your best bet is a Suica or Pasmo debit card, for sale at all stations, which doesn't save you any money but saves you from the hassle of buying individual tickets.

If your trips are limited to a certain area, you may want to look into the regional passes, too. For example, the JR East Pass:

https://www.japan-rail-pass.com/pass...FYkjgQodQtoFKg