Are we using "to jest" (rather than "ten jest" or "ta jest" because jajko is neuter? or is the form always "to jest"?
To jest = "it is" or "this is". Always. There is of course also "ten jest", but it translates more or less to "this one is".
You need to remember that after to jest you use the nominative case, hence to jest jajko.
Eggs are generally countable, and for the rare examples when it could be treated as uncountable we would probably specify it further.