TFTP is a protocol that runs over UDP/IP, so you need an IP network. A serial port by itself does not provide an IP network. To provide IP over a serial port, you have to run a protocol such as PPP.
how to tftp with telnet over wifi
TFTP and telnet are two separate protocols that run over IP and have nothing to do with each other. I don't know what you mean by running "tftp with telnet".
In any case, if your alternatives are between wifi and PPP over a serial port, you are probably much better off with wifi: it will be much faster because a serial port will usually only go up to 115200bps.
If all you've got is a serial port and you want to transfer a file, especially is busybox is involved and you need "stability" (not sure what stability problems you are experiencing?) then honestly XModem and ZModem will be much better options that running PPP and TFTP on top of that. Certainly it will be a lot easier to set up...