the phrase "Don't mention it" phonetically looks like [ doʊnt ˈmɛn ʃən_ɪt ] I think the primary stress is on the second syllable "ˈmɛn". Am I right? But my question is, is it important to add any stress on "don't" or is it optional? The last two syllables are connected together "ʃən_ɪt" and I don't think they need any stress. Any suggestion appreciated.
To mean thank you, the sentence stress is on the word mention, and the word stress in mention is on the first syllable: Yes.
It is not a very strong stress.
The final consonant (nt) and ("hidden") vowel sound ("tUH") in don't are unpronounced or weakly pronounced. It is of short duration, weakly pronounced, and the Tuh that ends it in some other patterns is usually not pronounced.
I find that there is a secondary stress on "don't" so long as the "n't" part is pronounced. In a sufficiently casual and reduced style, you can omit the "n't" and wind up with just "doe", and then there doesn't need to be a secondary stress: [dəwˈmɛnʃn̩ɪt].
If you are pissed with someone and they get a bit annoying with someone than you stress the don't, however if you are say someone like a teacher and you want to tell your pupils not to do something you wouldn't stress the don't.