When i have multiple tasks that reads out data from a variable, and only one task writes to a variable. Is it safer/better to use a mutex? It is about a 32bit variable on a 32bit processor.

One writer and multiple readers does not require a mutex, provided the write/read is atomic. A 32bit write on a 32bit processor should be atomic, so you are ok.

Take care that on some architectures this cannot be taken for granted. As an example on a Cortex you can have a 32 bit variable that is not 32 bit aligned, extra aligment definition may be needed. fwik on ARM7 it is always aligned, someone please confirm this ;)