Recently I have very often come accross with expressions like

                                  Her zaman çok iyi oldun.

                                  Annen nasıl oldu?

According to what I have learned I would use the ek fiil here: çok iyiydin, nasıldı. Doing so would I make a mistake?

This usage is mentioned in Göksel and Kerslake´s Comprehensive grammar: "In nominal sentences oldu does not always denote change in state (´become´ )." But I wonder, is it very usual, is it getting more usual?

Her zaman çok iyi oldun. You have been always good.

Annen nasıl oldu? How is your mom so far? (Has she got recovered from illness, etc.)

Her zaman çok iyiydin. you were always good.

Annen nasıl? how is your mom?

shortly, in first two sentences, there is present perfect tense meaning. actually there is no such tense in turkish but if we have to express that meaning we use like that.

Do you mean Her zaman çok iyiydin does not have the meaning ´you are still good´? I have understood (I)DI attached to nominals never denotes perfective action (i.e. action finished and done with).

If you want to ask ´how was your mother last time you met her´ what would be the most adequate translation?

My learner´s try would be: Annen, onu son defa gördüğünde, nasıldı?

Or: Anneni son defa gördügünde o nasıldı?

But of course, I´m waiting for İkicihan´s answer

Btw, thanks a lot, Abla, for interesting question, and İkicihan for no less interesting answer.

He wants to say it can give present perfect tense meaning. Action is still going on.

Translation: Annenle son görüştüğünde nasıldı?

my english is limited and grammatical terms confuses me.
i wait for some professionals to give better answers.

Your answer is fine. The present perfect interpretation never occurred my mind. It means ol- really adds something to the sentence and changes the aspect of the action.

Thank you.

Come on Abla. I remember the long threads we had discussed about -di having perfective meaning.

Güzel-di = It´s beautiful/nice (for example: The meal we have just eaten or the movie we have just watched)

Isn´t it called "present continuous"?

From wikipedia:

The present perfect is a grammatical combination of the present tense and the perfect aspect, used to express a past event that has present consequences. The term is used particularly in the context of English grammar, where it refers to forms such as "I have left" and "Sue has died".

This is only one of the uses of the present prefect.

When I´m asking "How long have you been waiting?", it implies that you are still waiting, unlike with "How long did you wait?". I think this is the use case that Gokuyum is talking about.