It can be unsightly and upsetting when a home in a neighborhood isn't being maintained like the others. It might be an overgrown yard, a fence in need of repair, paint peeling on the home or even a car parked in front of the home that hasn't moved in weeks.
I believe most people want to be good neighbors and may be willing to correct the issue once it is brought to their attention. In some cases, they may not agree with the same urgency and it might be necessary to seek other remedies.
The most expedient solution may be to contact the responsible person and describe your perception of the problem. An owner-occupant may be sympathetic to the neighbors and more than willing to correct the issue.
However, if you suspect that it is a rental property, check with the county tax records to identify the owner. They may be unaware of the situation and would actually welcome the "heads-up" to protect their investment.
The next step might be to notify the homeowner's association if there is one. The covenants or bylaws will specify how properties must be maintained and the association can enforce them.
The final step would be to notify the city for a possible code violation. Most cities have a separate code and neighborhood services division and some cities have 311 for non-emergency assistance.