It is a traditional fashion rule that polka dots and plaid should never be worn together, because the two patterns clash. In contrast to the idiom like oil and water, it is possible to mix polka dots and plaid, but it is taboo and foolish to do so.
Thanks to Damkerng T for supplying this example image (plaid shirt, polka dot skirt):
The first example states that the listener (you) is so elegant, graceful and chic that they can make even such an unfashionable combination as polka dots and plaid look good.
Regarding the ad, the suggestion here is that the 'polka dot' is someone quirky or unusual (possibly) in search of the same, because they are traditionally thought of as not pairing well with different patterns (e.g. plaid). Alternatively, the polka dot may be searching for their complement - someone who possesses the qualities they lack and vice versa - in order to form a complete or balanced couple, perhaps justified by the trope opposites attract. In the context of a personal ad, one is generally assumed not to be searching for someone who would be a poor match, as the phrasing might suggest. This is a very idiomatic construction with subtle, nuanced meaning which I find quite difficult to describe accurately.
As an aside, for the purpose of understanding the idiom, don't put trust in search results regarding the validity of wearing polka dots and plaid together. It seems that fashion's inscrutable caprice has brought the combination in vogue these days.