vocabulary
“I need some new clothes for the summer.” (Merriam-Webster’s Learner’s)
If the word, clothes, is plural, don’t you include a skirt into the clothes category? Can the speaker have in his mind for something singular, e.g.skirt; or just plural, e.g. pants, trousers?
As commented by @Walter, in OP's context, fancy is a synonym of imagine, believe. But the usage is becoming increasingly "dated"...
Many of the above instances will be the idiomatic exclamation "Well, fancy that"! (equivalent to "Imagine that!", "What a surprise!", "How strange!", etc.). In OP's context, Dumbledore could quite naturally have used any of those (possibly preceded or followed by something like "I didn't expect to see you here!").
Note that Dumbledore is an elderly British schoolmaster in a relatively "formal" situation (he's greeting Professor McGonagall, one of his staff who apparently happens to have adopted the form of a cat).
Idiomatic expressions as used by Dumbledore won't necessarily be so natural for younger speakers, and per @StoneyB's comment below, even Dumbledore is speaking ironically here. Brits in particular still use fancy = like, want, be attracted to quite naturally today, but fancy = imagine, believe, think is now largely confined to ironic/sarcastic contexts.
I absolutely believe that it's not the "big words" that make a good sentence or a good writer/speaker.
If you can clearly say what you mean, you are using the language well enough.
Or is the following sentence better? (Full of big words, basically the same meaning...)
If you can explicitly and concisely state what you want to convey, your usage of the language is adequate.
So, yes, go ahead, use whatever words you have in your vocabulary.
Best Answer
Clothes is plural because it includes plural items of clothing, which may be individually expressible as either singular or plural. Whether your shopping list has a) a skirt, a blouse, a hat and a raincoat, or b) a pair of socks, a scarf and a pair of slacks, each set may be referred to as clothes.
But if all you are buying is a skirt you don't say you need new clothes; you say you need a new skirt.