。
This guy's got a rotten personality, that's probably why he's unpopular with women.
Literal
This-guy [genitive-の] core-nature [subject-が] bad [because-から] non-popular is [conjectural-んだろ].
A harsh casual judgment. こいつ ('this guy') is the rough demonstrative-pronoun form of 'this one' for people, often with a dismissive edge — こいつ is blunter than この人, more casual than こやつ (archaic). 性根 (しょうね) is 'one's true nature,' 'core character' — the deep-down personality that determines one's conduct. 性根が悪い is a fixed expression for 'having a rotten personality/character.' 非モテ is internet/slang for 'unpopular with the opposite sex,' 'incel-adjacent' — 非 (not) + モテ (the noun form of モテる, 'to be popular with the opposite sex'). Part of the otaku-internet vocabulary family. だろ is the contracted casual だろう ('probably').