You need to keep in mind the relationship between the Yahoo Mail app and your online Yahoo Mail account. This relies upon a server (your Yahoo account, stored within Yahoo's online servers) and a client (the Yahoo Mail app on your phone). The issue to focus on is your online account is the primary, central source for all your Yahoo Mail activity -- all managed and maintained by Yahoo's servers while the Yahoo Mail app on your phone is just an email client that's constantly connecting to your online account for new incoming messages. When you send an email message using the Yahoo Mail app, the app is sending that data up to the Yahoo servers, and then it gets sent onward to the mail servers of whatever email service the recipient uses.
So your query is a conditional one -- using Clear Data on the Yahoo Mail app on your phone will delete your Yahoo Mail account that exists on your phone but it will not delete your online Yahoo Mail account. If you're referring to the account on your phone, than yes. If you're referring to your Yahoo account online, than no.
If you opt to re-use the Yahoo Mail app on your phone again later or want to use a different email app, you just need to re-enter your Yahoo email address and password info again.