Perhaps a combination of both. The gap is too large, the mitered corners are not a great look on flat casing. The heights don’t match. Does this look go with the other casing in the room?

we decided to change this casing because we are adding wainscotting in this room and we thought the old traditional casing we had wouldnt mesh well with the modern square wainscotting going in this room. This bay window is the only window in the room.

He’s certainly not a craftsman who takes pride in their work. Even brand new houses have slight waves and wobbles because they are built by humans, a good craftsman knows how to adjust trim to hide that.

Place a level against the window jambs please. Since the casing reveals appear to be consistent, I'm betting the window(s) are installed out of plumb.

He likely had a choice between maintaining consistent reveals on the window frames, or trying to line the edges of the trim boards up with each other. I'll guess that your old trim was maybe 2 1/4" width, so the gaps were large enough for the inconsistency to be less noticeable. It just so happens that you chose a size where they now sit much closer.

Personally I'd say that the fault here lies more with whomever planned this out than the execution of it, even though I'd agree that it could have been better. If your contractor was responsible for both, I don't think that it would be unreasonable to expect him to address.

As for fixes going foward, some filler of some sort in those gaps (not just caulk globbed in) would be one option and could look seamless after paint if executed well. Using wider boards ripped to the exact width needed would be a solution as well.

I agree with HomeSealed. I probably would have had the boards mitered on the long side and butted in the corners were I going to trim it with square edge boards. That tiny gap is going to make this a b****t to paint or stain.

I think in my head wainscoting is not modern but the wirndow trim is so maybe a peak at the waincoting will give this a better idea . I think the windows probably not plumb and so he did the best he could from your wants . Sometimes what you want and what you get are not the same. I think with a bit of finishing this could be fine .