I feel like Adventure Time went through a lot of phases. It was never meant to be a big serialized story. It was a weird comedic show told in episodic bite-sized chunks with dark undertones and hints at something bigger.
So when it started playing around more with lore and expanded on the overarching threats that people wanted, it had trouble balancing it with the zaniness and regular returns to the status quo. So some people wanted more episodic adventures, and others wanted long-running character development and bigger conflicts. Both sides had highs and lows, because they were clearly making up most of it up as they went along.
All in all, I do think it's had a good run, but it feels ready to end. I'll miss it though. There aren't a lot of shows these days with a setting and cast of characters with half as much creativity behind it.