Hi,
Thanks for your bug report regarding URPRenderer2DMeshEditor taking precedence over your custom editor.
This behavior is actually by design. The issue is not related to the isFallback property, but rather to the SupportedOnRenderPipeline attribute.
Internally, we prioritize user custom editors over our own internal editors. However, when an editor has a targeted SupportedOnRenderPipeline attribute, it gains higher precedence than one without it. In this case, the URP editor is winning over your custom editor for that reason.
To ensure your editor takes precedence, you should add the same attribute to your custom editor: [SupportedOnRenderPipeline(typeof(UniversalRenderPipelineAsset))]
Once applied, your editor will also have the higher precedence and should override the URP editor as expected.