Is it appropriate to say pressure is all mine when somebody says thank you?
No. The correct phrase is "the pleasure is all mine", but this
is not an appropriate response to "thank you". "The pleasure is all
mine" is a polite and gracious response that can be used when the
other person indicates pleasure. For example, if someone says, "It
was a pleasure talking to you" or "It was a pleasure meeting you",
you can respond with "The pleasure was all mine."
If someone says "thank you", the traditional response is "You're
welcome."