。
Sorry for being such an ugly-mug big brother.
Literal
Ugly-man [gen-の] older-brother [being-で] was-bad [soft-な].
The prefix ブ (originally 不 'not / un-' grown into a katakana intensifier) attaches to words like ブ男 'ugly man,' ブサイク 'ugly (face),' signaling a derogatory or comic caricature. 悪かった is the past of 悪い, here used idiomatically as an apology ('sorry / my bad'). The で after 兄 marks 'being / as (an older brother).' The sentence-final な softens the self-deprecation into a wry, mock-apologetic tone.