Could "μια" be skipped? (Είναι γενική ερώτηση)
Yes, it could be skipped. Thank you for commenting, I added it as an alternative translation. ^.^
Why not "Είναι ένα..."?
Because ερώτηση is feminine, so you have to use the feminine indefinite article μια (or μία, though that may be more commonly used for the numeral "one").
ένα would be used for neuter nouns.
How do I know when to use "μία" or "ένα"or the other ones?
ένας - one/a masculine eg ένας ελέφαντας
ένα - one/a neuter eg ένα βιβλίο
μία - one/a feminine eg μία γάτα
Why not αυτη είναι
Αυτή is feminine. 'It' is general purpose neuter and therefore 'αυτό είναι', although just 'είναι' is usually good enough for 'it is/he is/she is'.