、？。
Wo-would it be bad... to call you by your first name?
Literal
N-no-good [maybe-かな]? Bottom [gen-の] name [with-で] call [conditional-じゃ].
The stuttering だ、ダメ marks nervous hesitation. 下の名前 (literally 'the lower name') is the Japanese term for 'first / given name' — distinct from 上の名前 'upper name' = 'family name,' reflecting the order in which Japanese names are written (family name first). 呼んじゃ is the casual contraction of 呼んでは, the contrastive topicalization 'as for calling.' The whole sentence is the displaced-topic/afterthought pattern — the conditional hedge (かな) comes first, then the premise (下の名前で呼んじゃ).