Hearthstone always had some "silly" artwork in the past, as it still has more "serious" artwork in modern expansions.
I will concede that in quantity, there are more cartoonish or "cute" cards in modern expansions than in the past. Not a surprise though, as the game is actually intended to be more on the silly side, and not a perfect card-based recreation of the source material, like you have it in Gwent or TES Legends. It might also have to do with expansions themselves being generally more lighthearted (to varying degrees, of course).
I also see that newer cards usually have less contrast, but I can also see why the developers might prefer bright colors, when the cards are already a bit small in the game. When the artwork is too dark, going for a realistic look, it's also harder to make out the details on screen. It needs to be mentioned that older expansions commonly recycled old artwork, while new sets (well, "new" like since Old Gods) usually have completely new artwork.
I can see what might bother you. Still, your examples are cherry picking, and I could name dozens of newer cards that are not on the silly side. I can't argue with your impression, but it feels you are little unfair, and mostly upset that artworks featuring prominent characters are not matching your preferences.
Keep in mind how subjective the topic is. I know someone who had the exact same sentiments already during Descent of Dragons, that all the dragons would look "chibi" (yes, he seriously said that), and in all honesty, I can't even begin to figure out how Murozond the Infinite or Deathwing, Mad Aspect aren't enough "badass dragon" for you. Bright colors, but especially with Murozond, I think it actually enhances the otherworldly vibe.
I am totally with you, that art is an important part of video games, but even if we say that Hearhtstone has changed substantially, it's a different question (and probably comes down to taste) if it really has changed for the worse. Personally, I'm fine with it still. There are some artworks I like, some I don't, in every set new and old. There's enough variety that goofy gnomes and the like don't irk me. And there are only very few that really bother me.
Either way, I wouldn't discuss this in terms of "quality", because quality usually means craftsmanship, while you are (I assume) mostly lamenting the style, like the intensity of colors and shades, or the cartoonish look. But darker pictures are not automatically more sophisticated. If you don't like it or like is less, that doesn't mean "art quality is suffering".