Fair point, though I consider the RPG genre in general dull. I really, really don't like most turn-based RPGs for various reasons, and they have to have something that truly grips me in either story (which a lot of them don't) or, even more so, gameplay (which even more of them don't). More often than not, I will pass on RPGs because I'm normally a pure gameplay man which RPGs often don't provide in favour of their storytelling.
Undertale has the story aspect of it nailed the fuck down. To the point that I can get over my general disdain for RPGs. It is commendable that it can do this merely by its writing. And, as mentioned, the combat system is definitely unique enough to be quirky and funny and have me chuckle here and there, but overall it stands only marginally above other RPGs for me.
As an example for gameplay in a turn-based RPG I enjoyed I'd actually go and name Eternal Sonata, despite its fairly lackluster storytelling. It was the basic Namco JRPG combat, but it had enough twists to make it compelling for me, and as the game progressed, more mechanics would get added to the combat that would shake it up once more.
Undertale is trying to do this with the different mechanics like the blue gravity mode and whatnot, but it only applies to a handful of boss fights and just isn't enough.
As said, if you like the gameplay, more power to you. I am genuinely glad you get something out of it, and I'm not being sarcastic here. After all, I'm not saying you are wrong or any of the likes. It's just that the combat doesn't do anything for me (and apparently others as well if this string of postings is anything to go by).
Of course, we can argue the semantics, interpretations and dictionary definitions of words like "dull" or "engaging", but I don't think there's any sense in it. We can perfectly agree to disagree.