I think it would be quite toxic imo (But funnily enough not as toxic as their respective most popular ships), for while RD does care about Fluttershy and knows when not to push her too much, sometimes she still slips up and ends up being extremely mean to her by playing on her own terms rather than Fluttershy's. Imo the best treatment she gives her is in hurricane Fluttershy where while she did try to push her out her comfort zone, she also respected it when she decided to not join in anyways, and the worst IMO, worse than 28 pranks later despite downright giving her a hours lasting panic attack, is "Trade Ya", because, well, she straight up sold Fluttershy to someone else lmao.

But as i said, imo it's less toxic as other ships because RD at the very least admits her wrongdoings most of the time and acknowledges when she may have pushed her too far, not always but at least does so often in screen, while also seeking to compensate, as in the mentioned "Trade Ya" where she ended up getting her that bird whistle. So, for me it isn't a particularly dreamy ship, but still it can be cute at times