Nope. Not at all.
Personally, I don’t think isometric is too cartoony.
Those two pics are kinda cute. 
If you want to make it isometric camera style. That’s quite okay for the type tycoon/simulation type of game that you want to make.
The pics you showed me look like 2d images. But then again, it could be 3d models with Cel shaded graphics, or cartoon style graphics.
In the case of the pics I showed you.
“Especially the Donkey Kong tropical Freeze pic.”
You could make the game with a 3d isometric camera view style and have a semi realistic/cartoony graphical look, for your game buildings,enviroments etc.
Which might be more cutesy, crystally yet fun to look at.
Or if you desire, an isometric game, with a Cel Shaded cartoony look, similar to those 2 building pics that you showed me.
I would also suggest, a type of, isometric postioned player usable free to control moving camera, that can rotate around your city, scenery, or building landscape.
Or, if the game is isometric looking, with 3D models.
A camera that can move above, around, and snap back to default, isometric view(s) when the player is done rotating the camera. Or finished, looking at buildings or objects, behind your front objects, or further in the background of your scene.
I hope that helps.
PS: Only you alone know how you want your games graphics to look.
Whether it may be similar to DK freeze style or Cel shaded look, like in your pics.
Any one of those artistic Graphical looks, could work well, for this tycoon style game that you want to make.
I hope you achieve making your dream game!
Many Blessings and good luck to you!
