I'm well aware that there's more than one type of fanservice, but, form my experience, there's only two I've heard of. The first one is when you pander to your fanbase by having something happen in a show that fans would probably love, while the second one is the more PG-18, "eyecandy" type of fanservice. The former doesn't happen that often, nor is it exclusive to the girls (and the ones that are related to the girls usually happen after the series they were in ended, in episodes where they reunite with Ash and reminisce about their past adventures and showing off some of the popular strategies that they used). And the second one, according to all of the definitions I've seen, be they from Wiktionary ("the inclusion in a work of fiction of material, especially of a racy or sexual nature, which has no relevance to the storyline and is designed merely to excite the viewer") or TV Tropes ("the use of sex or sexualized situations to reward or entice viewers") or some other sites, is strictly about having a... ahem mature scene involving a character/group of characters that's supposed to "excite" the viewers, which is definitely something that rarely, if ever, happens in the Pokémon anime. The only two moments that I can even think of regarding the latter would be the Serena/Ash kiss (and even that's a stretch) and Lillie and Altaria's mud rubbing scene from the hot spring episode (which was pretty minor anyways and quite forgettable). Other than those two, I don't remember seeing this kind of fanservice in the anime.Fanservice could be pretty varied (and not inherently sexualized).
Even a cute girl wearing a pretty fancy dress while there is sparkle around her could be fanservice.
In itself, fanservice could be just a cute girl being cute.
By your logic, flowers and rainbows and everything nice (and maybe even little things, like a smile or an expensive-looking costume) would be considered fanservice.Sure, Dawn and Lillie are two of the best-written characters of the anime, but they are both still mostly there for fanservice. People like cute girls after all and cute things in general.