How to make a smooth serial print?
I bet your problem has nothing to do with “smoothness” (whatever that
means), but is entirely due to lack of message framing. The serial and
bluetooth links do not transmit messages: they transmit only a stream of
bytes, with no built-in notion of where a message ends and the next one
starts. Relying on the timing of the received bytes to tell the messages
apart is very unwise. You should instead use some kind of delimiter for
your messages.

For text-based transmissions like yours, the most natural choice is to
send a line terminator at the end of each message. Typically a CR+LF
pair. The Serial object has the println() methods designed precisely
for this purpose: they send their arguments immediately followed by
CRLF:

Serial.print(x); Serial.print(","); Serial.println(y);


At the receiving end, you have to buffer all the incoming characters
until you see the line termination. Then you know the message is
complete and you can process it.