This T20I series in July 2021, like the ODI series which preceded it, only taught us that:
India C is better than Sri Lanka.
Sri Lanka is better than India D.
It was the first bilateral series in any format which Sri Lanka had won against India since July 2008.
A summary of all T20Is between these teams:
22 matches. India lead 14-7 with 1 no result.
In India: India lead 8-2 with 1 NR
In Sri Lanka: India lead 5-3
Neutral: Sri Lanka lead 2-1
A summary of individual performances follows. The number of matches is not enough for meaningful calculation of averages, strike rates and the like.
Most runs (100 or more):

Dhawan moved into the top position ahead of Kohli. MD Shanaka has the most runs by any current SL player.
RG Sharma is the only one with a century, while Kohli is the only one with 4 50+ scores.
Highest innings score (50 or more):

From this bowler-dominated series there is only SA Yadav’s 50.
Most wickets (8 or more):

YS Chahal and PVD Chameera from the current series are at the top. Chahal is the only one with 2 four-fors.
Best innings bowling (includes all cases of 4wi or more):

While the overall record is still Ashwin’s 4-8, it was almost surpassed by Hasuranga’s 4-9 in this series. There is also B Kumar’s 4-22 from this series.
Most dismissals (5 or more):

Dominated by Dhoni, while HH Pandya has the most catches by a non-keeper.
Most dismissals in an innings (3 or more):

Dhoni again.
Best all-round match performances (Minimum 20 runs and 2 wickets):

Yuvraj’s 60* and 3-23 is clearly the best. No good performances in this series.