Skipping INVERSE TRIGONOMETRY will affect my JEE MAIN score or not ?

It will rarely cost u 4 marks

Are you targeting 2018 exam?

Below are the weightage wise topics for your preparation which you can't skip.

Do OR DIE TOPICS

Probability -around 7%
Definite Integration -around 6%
Circles -around 5%
Vectors -around 5%
Theory of Equations -around 5%
Sequences and Series -around 5%
Complex Numbers -around 5%
3D -around 5%
Matrices and Determinants -around 4%
Differential Coefficient -around 4%
Sets Relations and Functions -around 4%

IMPORTANT TOPICS

Application of Derivatives -around 4%
Trigonometry -around 5%
Parabola -around 3%
Limits -around 3%
Heights and Distances -around 2%
P&C -around 2%
Straight Lines -around 2%
Areas -around 2%
Differential Equations -around 2%
Continuity and Different-ability -around 2%

LEAST IMP.

Other Conics
Binomial Theorem
Mathematical Induction
Prop of Triangles
Statistics
Pairs of St.Lines