My, my, this is what they mean by 'even a horse groom looks fine in good clothes.' When you put on a suit, you actually look pretty sharp.

Literal

Oh-my, horse-groom [to-にも] clothes [quotative-とは] this-thing [copula-だねぇ]. Suit wear-[conditional-と] fairly cool [copula-じゃん].

馬子にも衣装 is a proverb — literally 'even a horse groom (looks good) in fine clothes,' meaning clothes make the man (i.e., anyone looks decent when well-dressed). あらまあ is a feminine exclamation of surprise. じゃん is casual contraction of じゃないか 'isn't it!' The speaker is pleasantly surprised at how someone cleans up.