Some mixers will work much better than others. You need an mixer input with very high impedance, like 1 Meg or higher (and usually labeled as 'Hi-Z' or 'instrument input' or similar). The problem with many contact pickups is that they have a high output impedance. Plugging them straight into a standard mixer input (often around 100k or less) loads the pickup down and gives you a thin, nasal sound with low signal output (low level). My Mackie Onyx has the first two inputs a with Hi-Z setting that makes a contact pickup sound great. The other inputs don't sound nearly as good for that purpose. But when I play out (and not through my own PA) I use an LR Baggs Para-DI that gives me a Hi-Z input to terminate the pickup correctly, adds level and frequency control (+ a notch filter, good for cutting out feedback in different venues). It also has an effects loop for other pedals, which I hardly use.