I know I can use Tikz and insert the source code in TeX. But is there a better way to do it which has a better user interface?
LyX has good integration with Dia. Go to Insert > File > External Material and change the template to "Dia diagram". It is nice to have the integration because LyX knows how to convert a Dia diagram on the fly. So if you edit a .dia file you don't have to remember to re-export it to the corresponding format (because LyX does this automatically for you).
So you can just include a Dia diagram in LyX. And anytime you want, just right-click and go to "Edit Externally". When you save your edits from Dia, everything is taken care of.
Another advantage is that there is a preview in LyX of what your .dia diagram is.
You can use any tool with a graphical user interface and export an image file for use in LaTeX (and LyX). An example is Inkscape, available for Windows/Mac/Linux, which can be used for block diagrams. Export to
Some more options, (including Tikz-related tools), are mentioned in What GUI applications are there to assist in generating graphics for TeX? and https://en.wikipedia.org/wiki/PGF/TikZ#External_links.
This is an addition to the already good answer of Marijn.
Direct answer to your question
Maybe have a look at TikzEdt. TikzEdt is a combined WYSIWYG/text editor designed for editing Tikz code [taken from the website].
Indirect answer to your question
Also, you can use other tools like Microsoft Visio (offline) or draw.io (online).
The already mentioned similar question: What GUI applications are there to assist in generating graphics for TeX? leads to more tools: