Don't listen to them. You're getting really bad critique. Your framing at times is poor, but image quality is perfectly acceptable. I see nothing wrong with noise in your images. Some problems are caused by image compressions, but I also don't think you need to correct it, you upload in whatever size you wish to upload, and use iso that you wish to use.
In outdoors shots you've wind, and It's better to have higher iso and no motion blur than go for low isos and blured image. Anyway, if you really wish to go for smoother images, look at low iso, and look at your processing. If you've no wind, you can go as low as you want and by stabilizing your cam firmly go for long exposure, and still get smooth shots. When you've wind, you need to take test shots, and see at what shutter speed you get no motion blur. You'll often have to open your aperture as much as you can. Personally I think aperture and shutter are much more important creative tools, and I'll gladly bump iso into 4000's and deal with noise if need desires smaller apertures.
Things that cause noise on editing side, are compression used, raw conversions, colour spaces, and sharpening. And it's being negated by blurring techniques and noise removal, and noise adding tools. Work on raw, in cam noise reduction is often crappy, thanks to cam needing to save on processing power. Sharpening is a major factor. To avoid it, ether use selective sharpening and only sharpen details, or use masking to achieve the same. Whatever you do, be very careful with noise reduction. Too much of it and you will kill details, and introduce way uglier artefacts. And finally, add noise! Fake noise looks prettier, removes banding, and other imperfections. First denoising and then adding tinny amount of noise is well established practice.