I'm not entirely sure why there seems to be such a divide about the game's singleplayer content. They cut a lot of the chaff (home run contest and other silly mods like the target test from 4 and the trophy rush), removed the terrible Smash Tour and mediocre Smash Run, and merged Event Mode with a variation on Subspace emissary.
There's really only two major singleplayer modes (World of Light and Classic Mode), and both of them are great. Classic mode is by far the best it's ever been, and while I do admit that I miss the platforming sections and the connective tissue that was the cutscenes from Subspace Emissary, I think that what they did here was refine and coalesce multiple modes and features into what I feel makes a lot of sense in the context of the world.
IT's not a perfect game but it's easily the most fun to play solo of the entire series. The original, Melee, and WiiU/3DS were not great to play singleplayer modes outside of Event Mode, and while I personally loved Subspace I think that's a mixed bag, too. So no, I don't think this has lackluster Single Player modes, I think it's a well-refined version of the best singleplayer content available.
My Console Library:
PS4, PS3, PS2, PS1, WiiU, Wii, GCN, N64 SNES, XBO, 360
3DS, DS, GBA, Vita, PSP, Android
Top 6 this generation:
Bloodborne, Sekiro: Shadows Die Twice, God of War, The Legend of Zelda: Breath of the Wild, Dark Souls III, Red Dead Redemption II, Rock Band 4