Hi there,
Getting to Kawaguchiko unfortunately is not that straight forward. There are two main options to consider:
1.) Take the Shinkansen from Osaka to Mishima station, from there, express busses run to Kawaguchiko, see (https://www.fujikyucitybus.com/en/mishimaliner/). The bus is not covered by the JR Pass.
2.) You can take the Shinkansen to Shin-Yokohama station, then local JR Lines to Otsuki station and finally the Fujikyu line (not covered by the JR Pass) to Kawaguchiko. This latter option is more cost efficient when using a JR Pass.
The fastest option depends on how long your transfer times are, unfortunately the bus at Mishima sometimes has departure times that don't match with Shinkansen times, making for a long wait.
Hope this helps,
Daniel