Open source software that I hate. Must we use blender to make mesh objects?

Is there an all alternative?

Well, as advertised on the imvu.com creator landing page, you can use any 3d editing software of your choice, they specify : 3ds MAX, MAYA, Blender and Sketchup...but you can use absolutely anything as long as you are able to export a FBX file at the end.
Now the real issue might be, for new people that will need tutorials for creating into imvu standards, mesh naming, materials and what not, that those will be scarce in different softwares on inexistent.
Everyone's best choice is indeed to use someting supported by the imvu creator community as you will inadvertentedly run into issues and you want to be able to find resources and people that can help with that in the community.

PS: What seems to be the issue with Blender ? you really made me curious about "hating it" , most people are just better accustomed to something else..but never heard of anyone strainght on hating it.

If you want to go real 90's old school, slimed down, and simple: https://www.anim8or.com/

^^ There's converters by boris that let you export from anim8or to IMVU.

The blend file is the major file format used by the program. Blender uses this format to save nearly everything including objects, scenes, textures, NURB objects, lightning data, vertexes, sounds and even last settings of user interface before the program is closed. Updated: June 9, 2020 The default software associated to open blend file

Source: https://www.file-extensions.org/blend-file-extension

If you want to use blend files, then you have to use blender.

When meshing I go for blender and for animation I used blender, 3ds Max, Motion Builder, iClone 7 and its pipeline.

I worked with Unity and a few others.

Blender is fun to play with i wish i could learn to use unity better!

Blender is really good. Id been learning to used it for 3 months from katbits youtube tutorial.

im just trying to learn blender, its a very creative application

Also trying unity now.