Again, from what I've played from both series and what I've heard...
I think Paper Mario's problem is related to the fact that it's experimenting a bit too much; I've seen this with a couple of other game series too. Like, Thousand Year Door was an upgrade to Paper Mario in some ways (such as the partner system getting an update), but since Super everything's gotten different in each instalment, and that means that rather than taking the strengths from the last one and fixing up the weaknesses, they've got to create a new system that they think will do both, and that's not been tried and tested so it's not guaranteed to work. Again, I liked Super Paper Mario - it was different, yeah, but whilst I have mixed feelings on the 3D gimmick, the platforming-RPG hybrid direction it took was still fun and the writing was very enjoyable (and the music is fantastic, probably my favourite of all the Mario RPGs in fact). But none of that even stuck, did it?
The one thing I can say that might be good about Colour Splash being similar to Sticker Star is that if they take notes from complaints about Sticker Star - the lack of memorable NPCs, the barebones plot, the battles being useless, the confusing puzzles - and fix them, then they'd be creating a new direction for the series that, while it wouldn't be the same as the original RPGs they were, would at least afford it an identity and allow it to grow in quality rather than the meandering it is now. Granted, I don't know if Colour Splash is actually doing those things...
Meanwhile, having only played Bowser's Inside Story and Dream Team Bros from the 'pure' M&L titles, I can tell you that I don't believe the Mario and Luigi series to have the same kind of problem. Yes, I found Dream Team in the end to just merely be 'okay' (I found that it was way too long to the point of feeling padded and tedious, the dream world and Luiginary works were a little boring and both the Giant battles and the new 'running away' mechanic in normal battles were annoying), but in the end it was still Mario and Luigi. The quirky new characters and humour were still there, the interactive RPG battle system was still there, the music was still there. So it was easier to simply hope that the next instalment would be better - all of the basic elements I liked were still present, after all, so it would be difficult to make something that wouldn't at least give me what I was expecting. Paper Mario fans don't seem to have that luxury any more.
Edit: As for Paper Jam? I echo R_N. It's got the Mario and Luigi gameplay so I can't fault it for its mechanics, but the writing seems to be the slightly weaker variant we got in Dream Team. I got to the desert level and just haven't been grabbed enough to go back to it yet. It's still an alright game, but there are far better ones in both series proper.