Starter forms I've been disappointed with? ...eh, I can't think of any, really. Probably the closest I've come to "dislike" is with the Tepig line.
Like, it is unfortunate that they went with the Fire/Fighting combo for the third time in a row. The previous two generations, it felt like it was more for early-game balance than anything--y'know, with both Torchic and Chimchar evolving the earliest at level 14 and gaining their secondary typing (and first Fighting-type move) so they'd have an advantage against the Rock-type first gyms. (It's the same reason they've had Charmander learn Metal Claw early on since Gen 3, actually.) And since we obviously didn't have to deal with that this gen, that type combo on a starter feels like a bit of a relic.
At the same time, though, it kind of...fits, somewhat, once you learn a bit about its inspiration: specifically, that it was apparently inspired by one of the Four Great Classical Novels of Chinese literature,
Journey to the West. Fun fact: the Chimchar line (or at least Infernape) was borrowed from the same source; Chimmy's line was apparently inspired by the monkey character Sun Wukong, and Tepig's line (or at least Emboar) by the pig-looking Zhu Baije. From what I know, both characters were very much martial artists, thus justifying the Fighting-type on both pokemon lines. For me, that kind of thing's pretty cool, and the knowledge helped mitigate my initial disappointment with the type combo.
So yeah, even with that, I wouldn't say I'm
disappointed, per se. Really, my "dislike"--more like indifference--towards the Tepig line comes mostly from the fact that I'm just not that big a fan of pigs as much of anything other than a food source.
I still plan on using one in-game eventually, though. Who knows; maybe it'll grow on me through gameplay. I mean, hey, it happened with both Swampert and Infernape, so why not?