Quick answer: Not all lumps in cats are dangerous, but any new lump should be evaluated by a vet — distinguishing benign cysts from tumors requires examination and usually a needle sample.
Lipomas (fatty deposits), cysts, and abscesses from bite wounds are common and often benign. Abscesses are particularly common in outdoor cats — they're usually soft, warm, and may rupture on their own. Even benign lumps should be confirmed by a vet.
Any lump that grows rapidly, is hard and fixed to underlying tissue, has an irregular surface, or appears in a cat over 10 warrants prompt vet attention. Mammary tumors in older unspayed female cats are common and often malignant — lumps on the underside need immediate evaluation.
A rare but serious tumor can develop at vaccine injection sites (usually between the shoulder blades). Any lump at a previous injection site that's larger than 2cm, persists more than 3 months, or grows should be evaluated urgently — this is a specific cancer type that requires early action.
Don't take a wait-and-see approach with skin lumps. A fine needle aspirate at the vet is quick, minimally invasive, and can provide an initial assessment within minutes.