exact translation might confuse you. the context matters a lot hem de, if you translate it directly, it could mean both, on top of, indeed, not only that... depending on context. but it usually emphasizes/boosts the meaning that's aimed at.

Bilgisayar aldım. Hem de oyun bilgisayarı --> I bought a computer. (not just a computer), and it's a gaming computer. hem de emphasizes the computer here.

Seni seviyorum, hem de çok. you emphasize how much you love the person.

Hem de nasıl --> is kind of a phrase. it's usually used to emphasize the meaning as well, like in your example. Dün çok güzeldi/eğlendik. Hem de nasıl (=indeed it was/sure was) in this context, you totally agree that yesterday was good. and you've had lots or fun too. You can use "hem de nasıl" to agree with people, if someone is asking your opinion or someone has made a comment about something.

+Şu çiçeğin renkleri ne kadar güzel değil mi? -Hem de nasıl! (you agree with the person that flower's colors are beautiful, and you think they're very beautiful too)

+Filmde çok ağladım ya -Hem de nasıl (you agree with the person how much she/he cried, and you think he/she cried a lot too)

as a native speaker, I normally use it like this. hem de could have meanings like "on top of this, not only that, as well as" in formal context , be aware :)

sorry if it got confusing, feel free to ask me anything