The mistakes that make a backyard look tacky are usually excess and mismatch: too many small decorations, dyed red mulch, mixed paver styles, and features that don't relate to each other. One gnome is a joke; twelve gnomes, a windmill, and four styles of solar light is visual noise.
Material mismatch is the quiet offender. A gray concrete patio, tan retaining wall blocks, and red brick edging all in one small yard read as random even when each piece was expensive. Pick two hardscape materials and repeat them. Same with color: bright white plastic furniture and fencing yellows fast and fights with everything green.
Scale errors finish the list, like a tiny fire pit ring floating in a big empty lawn. Checking the whole picture on a photo of your yard with AI landscape design catches these before you buy.