There are about 7 different versions of the 70-300 lens.

Which one you have greatly influences my opinion of it.

The 70-300 G VR is a great lens. The newer 70-300 AF-P lenses are by all reports excellent. The others range from mediocre to terrible.

Looking at your first image of the helicopter I see several things wrong.

#1 - you say you shot it at 155 and cropped - why? It's a zoom, use it.

#2 - ISO 3200 - on this camera it's not really gonna give much good result, especially if you have to crop.

#3 - It looks like it's a bit out of focus. Ignoring the other two, this is a deal breaker. The D90 while it was a great camera, it does not offer the option of fine tuning autofocus. I can't guarantee it is needed just by seeing a few pictures but so far I'm not impressed.

#4 - the white balance looks screwy.

Looking at the second picture of the...knife and whatnot

#1 - where's the focus? I'm having difficulty finding it.

#2 - I can't really offer any further advice as there's no exif data/shooting settings mentioned for this image.

Summary -- depending on which 70-300 you have, it could share part of the blame. In either case, there's both an old camera issue happening here, and some misuse and incorrect settings being done as well. Also keep in mind that long lenses are not really intended for distant subjects. They're better suited to resolve more details on closer ones. Especially when you're talking about budget zooms like this. They really don't do well with atmospheric distortion over any meaningful distance.

Resolution - Get back to basics and shoot some serious test images with this lens with attention paid to the fundamentals. Get the white balance right, get the focus right, get the framing right so you don't have to crop. Reassess performance after this. If the 70-300 is the G-VR one..it's fine. If it isn't, I'd replace that before I bought a new camera. If it is the newer lens...I'd be looking at a used/refurb D7200 or better.