It's fine to not like it, you should just be able to explain why.
"Boring and dull" isn't a really good explanation, it can kinda be applied to anything. It's a very subjective criticism and doesn't do a good job of expressing what exactly about the game makes you not like it.
I love it because I feel like the vibrant and colorful retro graphics combined with the interesting designs for characters and monsters you fight, as well as the witty and warmhearted humor/general writing combine for a really amusing and charming experience--but on top of that, the game can deliver some pretty intense feels depending on the choices you make.
One of the criticisms I have seen of the actual game
is that the moral choices you make aren't as innovative as people make them out to be, which may be true, but I don't think that makes them any less interesting/exciting to make. The game also goes a long way to acknowledge the things you've done on multiple playthroughs which feels nice. It gives the game a lot of replay value despite being barely five hours long on an average playthrough.
Undertale is in my top 3 for this decade (the other two being GTA V and The Witcher 3).