I don't really dislike any types, but when I'm specifically asked what my LEAST favorite types are there's a few that always come to mind, like...
Rock: Honestly, it's just the fact that Ground and Steel exist. Between the two of them, Rock feels like it stands out a little less. I still like that it exists, I KNOW it's good competitively, and there's a lot of Rock types I really like... but as a type it really just doesn't stand out to me and feels sort of... meh.
Flying: Love the concept, but there's a few problems here. For starters, it just doesn't feel like its own type. Only ONE Pokemon in the entire series is PURE Flying-type, and only one dual-type line is Flying primarily. Everything else has it added on as a secondary typing, like it's just an extra 'trait' or ability, rather than its own thing. There's also the name... I just find it silly. Why is it Flying? There are several Flying types that don't know how to fly at all. "Sky" type would make a lot more sense, or "Wind" type... since it really seems to be based on wind.
Ice: Most people who know me would expect this to be my FAVORITE type... but sadly it's not, entirely due to the potential it has. The problem is... Ice-types have very few uses because as a type itself it's awful defensively. Sure, it's nice to have STAB on an Ice move since Ice is so good offensively, but... it's usually just not worth it. What's worse is most decent Water-types learn the best Ice moves, and Water is an amazing defensive typing. I love Winter, snow, ice, and the cold as concepts and real world things... but in Pokemon it's just too under-utilized and at too big a disadvantage. I also dislike a large number of Ice-types' designs. There are some I really like, but... most of them just don't work for me or even irritate me a bit.
Normal: Plain and simple? It's boring. I like the idea behind it, and technically it's a really good type... but it's just kind of dull. It's like "Original" flavored potato chips. They're serviceable, there's nothing honestly terrible about them and they CAN be really good... but even when they are they're just so... ordinary. This, paired with the fact that their one weakness, the types that resist them, and the type that's immune to them are all VERY common, and since Normal has no resistances, that ONE immunity to a fairly uncommon offensive type doesn't really come in handy that often... especially when most Ghost types learn a lot of moves of other types to entirely compensate. I also don't care for most Normal-types' designs. There are a few I love, but most of them are just extremely forgettable to me... especially after a lot of them went Fairy.