A few samples of your BEST photos may help us comment better.
If flash is not used, cellphones benefit from good lighting. Ensure your subject is well illuminated.
"Camera" motion must be minimised. Holding your breath and consciously steadying your arm helps image quality when light is not good.
ENSURE that your subject is in focus. Out of focus photos look terrible except when the effect is used for a specific purpose.
In portraits have the subject fill a significant part of the image. Sometimes the background is such that having the person involved 'small' is acceptable but, if the photo is of a person ensure that the photo is of the person.
Try to avoid having light sources behind the subject. If the background is brighter than the subject you will tend to obtain silhouettes.
Ensure the subject is looking at the lens. This does not matter as much if they are a reasonable distance away but, when taking closeups or selfies with two people, look at the lens and not elsewhere.
When taking portraits at medium to short distances ensure they are looking AT you (unless a looking-away stance is intended). If there are two or more photographers, if they are not looking at you but at another camera then it is often very obvious in the photo.
Identifying what it is that people say is wrong with your photos is a useful step in improving them.
THIS is one of my "Random Strangers" albums. The photos are not designed as 'works of art' (if any happen to be, that's a a bonus :-) ) - they are largely photos which happen 'along the way' as part of my trip-record / life-record. People are generally happy with these photos of themselves when they see them. Have a look - decide if any look like photos that you would like to have taken. (Ignore the rest :-) ). If you like them, work out WHY - how do they compare with yours? What can you do to make yours look more like the ones that you like?
Screenshot from album - much more detail in album proper -
It's not easy to properly simulate reducing exposure and then using flash - but this is a very (very) rough simulation: