This is my go-to method to decimate all kinds of CC. It's very clean and you can exactly decide where you want to keep denser geometry and where to reduce!
To make this process faster, there is a nifty way to select every other edge loop to quickly decimate the loops by 50%, which is especially useful for clothing meshes that have a tubular shape (use it on sleeves, pants, tops, shoelaces, jewelry chains...)
Remember to first convert the mesh into quads using alt+j as bellakenobi mentioned or it won't work.
Depending on the structure of the mesh, the loops might not get selected all the way through automatically and some manual selecting is necessary to finetune the result, but it's worth it.
I don't find this method takes an unreasonable amount of time, and it gives such great and clean results that it makes working with the meshes much easier too since you do not get faces randomly deleted like with the decimate modifier.
This is a helpful video tutorial that explains the technique: how to select every other edge loop!!
Once the loops are selected according to the video tutorial, you just dissolve the edges (x -> dissolve edges). Remember to also dissolve some of the edges going into the other direction (if you dissolved the vertical loops, also dissolve horizontal ones) to reduce overall density!
As bellakenobi mentioned, with one or more parallel loops selected, you can double tap g to enter edge slide mode to re-adjust the remaining edge loops. This is not only very convenient and less prone to cause issues, because it just slides along the surface of the mesh, not elsewhere, but edge slide will automatically adjust the UV map for you too to reflect those changes, which means less work for you.
Tips for fixing topology:
Selecting edge loops works best on quads, but there are several ways that you can edit and alter a face. for example, by selecting two vertices and pressing j, you can quickly cut a new edge between the two vertices. It's like a quicker version of the knife tool (which is very good to use too!).
With the dissolve options (press x), you can delete unwanted vertices, faces, or edges, depending on what you want.
I sometimes dissolve several faces next to each other and then cut out new edge loops in a messy area. Just experiment a bit!
It helps to periodically triangulate (ctrl+t) some areas with n-gons (faces with more than 4 edges) when you dissolve a larger area of faces to "settle" them so you do not lose the overall shape of the mesh. You can then convert the area back to quads afterward (ctrl+j).
After these edits, you may want to reset the normals in areas with dissolved topology since they may get wonky in the process. But with Mesh->Normals->Average->Face Average you can reset them and make them smooth and gorgeous again.
A really helpful tip is to look at the Blender manual which explains a lot of techniques and provides shortcuts, or videos that explain small features that can have a big impact, speed up your workflow, and improve your meshes.
I recommend BlenderSecrets, a YouTube channel that offers lots of quick tips for Blender that make life easier.