I'd recomond getting on since it what offical competitions use. And it's good practise. However it's up to you. If you want to get good at 2x2, pyraminx or skewb use one.
i have a yuxin timer it works and all that but i usually prefer having the inspection of cstimer than the accuracy of a stackmat although it is usefull for 2x2
Using a stackmat can add 0.5 seconds to your solves compared to a spacebar on a keyboard, but this difference disappears as you get used to a stackmat. If 0.5 seconds is a significant (>5%) portion of your solves, then I recommend using a stackmat, otherwise there's not much benefit to it.
Don't get a Yuxin or YJ. Just don't. If you have the money, I recommend getting it. Especially if you want to do Pyra, skewb or 2x2. And 3x3 to an extent. Plus, you should be doing unlimited inspection at home.