I think he looks even cooler because he doesn't even try to dress up.
He looks even more stylish because he doesn't care too much about what he looks like.
ーI think he looks even cooler because he doesn't even try to dress up.
「彼は着飾らないからこそ、もっとかっこよく見えるんだと思う。」
looks even cooler で「もっとかっこよく見える」
dress up で「着飾る」
ーHe looks even more stylish because he doesn't care too much about what he looks like.
「彼は見た目をあまり気にしないから、余計おしゃれに見える。」
even more stylish で「余計おしゃれ・かっこいい」
ご参考まで!