@EarthboundBenjy the funny thing is the mobile versions were exclusively pixel art.

These graphics only happened with the ps4/3ds release in 2017 of this collection in japan.

Thinking about it, rather than an executive decision, I think this decision was made(particularly for monsters sprites) -because of the PS4 release. Since the PS4 is a TV bound console, most people who own one generally own widescreen TVs that are probably on the larger end of things. Over such screens, they would likely have stretched the pixels of monsters art to the point of looking very badly dated, especially in proportion to the traditional-styled UI(another pixelated game, Octopath Traveler, probably alleviated that with it's smaller UI allowing for larger monsters sprites that could thus have an interesting level of detail even if featuring a pixelated look).

Had this collection, back in 2017, been originally exclusively for 3DS(and thus exclusively for its smaller screen), I doubt we'd be looking at this "compromise"(after all even the Switch has an handheld option over whom's smaller screen the original monsters sprites would have been okay).

When it comes to monster alone, the art is not bad because it's just literally the concepts as they were literally drawn by Akira Toriyama himself(it's particularly notable if you ever compare with Toriyama's original concept drawing for Baramos).

Where the style fail the most is in the field sprites because these sprites were somehow updated yet their surrounding environment wasn't.

This said: it's just a suspicion from me but I suspect that with Dragon Quest Monster Retro's announcement, the trilogy release might well be the last of these "re-release" to feature such comprised graphics.

Because DQMR simply resolved the "how to fit stuff on a larger screen" by simply keeping the original graphics and ratio display and adding a background wallpaper in the remaining space over which elements of the UI can be overlaid for added use. An approach I think it might have learned from Collection of Mana and then expanded upon with these customizable UI elements.

It was unlikely they were ever to remake this trilogy in that style when they already existed in a form two years ago they could port over to Switch, but the precedents set by Collection of Mana and soon Dragon Quest Monsters Retro make me suspect they might be showing what future remakes/rereleases of "classic" titles might be like. So I have high hopes for potential rereleases of 4-5-6 (likely using the DS-era graphics) if they decide to further go with that approach since I feels it has potential(especially if they further build on that "background wallpaper but with added customizable UI" approach).