I wasn’t familiar with that onomatopoeia, so I tried to search around, but unfortunately couldn’t find it listed in the lists of onomatopoeia that I found. When run through MTL, Google thought it meant fluffy while DeepL thought it meant flabby. I am used to seeing ふわふわ (fuwafuwa) for fluffy, so I don’t think that is it. However, I am at the limit of my ability to figure it out.

The best I can make of it is through deepl: it maps ぽやぽや to “carefree”, “carelessly” or “pampering”. “すぐ” precedes it meaning “immediately”, “at once”, “right away”. Taking in the context: it is done on paper and not digitally, and that there is some whiteout at the bottom. Also the hair colour goes outside the lines. All this combined makes me think that it means that the drawing was done “from a loose hand”, meaning with some pace and without excessive construction like a detailed sketch. I don’t think it is done completely without sketching though, as the top left strand of her right (our left) twintail there seems to be a sketchline the inked lines.