Resource smoothing is very important in a project, there will be negative output if resources are not managed properly. List those negative effects?

Over- or under-utilisation of resources and labour. If you have labour shortages, you may not achieve necessary KPIs, miss deadlines and cause issues for next stages within the already tight schedule. Shortfalls in one task, with over-resourcing in other tasks would be a key identifier of poor project managmenet. If you have too much labour, you may be leaving other tasks short of labour, missing opportunities to get ahead, or just wasting money and risking demotivating your teams.