Not going to say that the PC-Engine is not a good machine, but the Bomberman 94 Genesis port has maybe less than 32 colours on screen. The console is able to manage 64 at once.
In terms of pixel art and colours, there is no reason the Genesis could not output this (or something very close) :
It's not just about what was the max colours on screen, it's about how they got them on screen. The key is 'up to' 64 colours on screen.
Playfields are parallax, and the MegaDrive could only use so many colours per layer. Something like Sonic was very clever in how it used them and the colours it used. You could in theory overlay them to get it looking like there were more, but for something like Bomberman that may have caused problems with collision detection and things and it would have been a nightmare to rework all the graphics into being split up in certain blocks of colours and put back together again.
Could they have got it looking a bit better if they'd started from scratch and built a MegaDrive game for it? Maybe. But for a game like Bomberman that doesn't require parallax the PC-Engine was really the ideal machine for it. And it's no surprise that the best versions were on it. I don't know what the exact number of colours '94 used is, they just used whatever they wanted and it looked great. If Mega Bomberman had looked better it would have been because it looked different.
The fact the PC-Engine didn't have parallax makes it all the more impressive when you do see it. I mean look at Lords Of Thunder above, that is insane and the trickery is off the charts. And the visuals fell apart on the MegaDrive trying to do it.
Mega Bomberman was still a great game, but '94 is still where it's at to this day.