Hi! I'm not aware of one at this time (which isn't to say there isn't one, but i haven't heard of it).  Unless this changes, i'd probably use a generic, well-featured pixel art tool like Aseprite in indexed mode, and rely on converting the assets instead. 

You can use NEXXT *somewhat* for simple 2bpp mode background graphics for the SNES too (for example mode 0 graphics, like SMW uses), but you can only edit one layer at a time then. And you need to supply your own system palette to replace the default one. 

YY-CHR has multiformat tile editing that works for the SNES as well, but it doesn't have the best layout capabilities, so it's virtually no good for level design. 

I might expand NEXXT to deliberately cover some areas outside the NES in the future (that doesn't act counter its main objective interfacewise), but for now all attention is on making it better for NES graphics use. 

Unfortunately, this a bit out of scope both for NEXXT, and for my own personal expertise. 
The only further advice i can give would be join the SMW central community, and to browse through its resources: https://www.smwcentral.net/