its generally hard to say what makes a beatmap good since people have different taste.
Generally speaking tho, nowadays mania community are looking that is uniquely mapped, Heavy SV usage, or anti-meta maps (non 4k-7K)
For example that is uniquely mapped, example could be ALEFY's recent ranked map
https://osu.ppy.sh/s/595792 with its unique usage of Long Notes on the highest diff
https://osu.ppy.sh/s/590341 sheriruth with its complex use of chords and LN combination
or maybe
https://osu.ppy.sh/s/588066 Re/Im with its heavy usage of SV throughout the last diff
more importantly, if you wanna know that your map is good or rather, fun, you should testplay the map yourself many times. from there you can have ideas how to make a fun pattern out of it
on a side note,
t/439430 this thread is kinda helpful for mania mapping