Translation:The food of my mom is usually very delicious.
It sounds more natural in English to say "My mom's cooking..." instead of "The food of my mom...".
"My mother's food" is rejected as wrong. And "usually is" is wrong but "is usually" is right. GEEZ DL, get a grip!!
Mum is used by Brits, Australians and New Zealanders (I also hear Canadians alternate between this and mom). Mom is mostly American.
"thức ăn của mẹ tôi thường xuyên rất ngon" means that her food is very often delicious but might sometimes be so-so. on the other hand, "thức ăn của mẹ tôi thường rất ngon" means that her food is very good on a regular basis.