I got gifted a small PC with AMD 5600G, installed Dolphin 5.0 recently to run Mario Kart, the internal resolution is 3X(for 1080P), but it runs slightly slow and the frtames is only30-40fps, made me really confused.
Is AMD ZEN3 so poor for Dolphin?
Try running a newer version of Dolphin. The 5.0 build was released 5 and a half years ago, and it has none of the optimizations, improvements, and bug fixes that have been made since then. The speed difference is huge compared to the latest development or even beta builds. So, first and foremost, see if a better, faster version of Dolphin solves anything.
Anyway, regarding the AMD 5600G, it should be a great CPU for Dolphin. However, keep in mind that the bulk of Dolphin's workload is focused on the CPU. If you can run Dolphin at fullspeed at native resolution (1x IR, the default 640x528) than that means the CPU is handling everything just fine. If you start increasing the resolution, you increase the load on the GPU. If you notice framerates dropping as resolution increases, that means Dolphin is demanding more than the GPU can handle.
Most dedicated GPUs these days will easily hit 1080p, even a bunch of Intel's latest run-of-the-mill IGPs run 720p without issue. The Vega 7 should be able to hit 1080p. This is my assumption however, as I'm not too familiar with the most current IGPs out there. As I said, try a more recent version of Dolphin and see if that plays better.
In addition to that, Ryzen like fast memory in dual channel. These is worsen with igpu also heavily influenced by the same factors. It possible that the memory in the system is in single channel(or only have 1 stick). A lot of prebuilt tend to use only 1 memory stick . While it might cost a little bit adding a other memory module or check if there 2 if they are install to benefit from dual channel memory. If you running 8gb of ram you would likely benefit from 16gb at some point at some point. You igpu is likely to be affect more by this. It might seem small but my main laptop using a i7 7700k and intel hd 630 the extra memory stick did make a decent difference for the integrated graphics, if I remember right the igpu was able to do 2x in mario kart and the vega igpu should be decently faster. The laptop also has a gtx 1070 also but it wasn't affect by this if that used nearly as much.
I use a r7 3700 and r9 5900x and both have no problem with mario kart with reasonable graphic card (gtx 970 for the r7, rtx 2070 for the r9) or most games outside of the most demanding games like rogue squadron 3 and the like. So the processor using zen 2 or 3 isn't the problem. Core count wouldn't matter either since even 6 core is more then enough also.