C (clang), (削除) 82 (削除ここまで) 78 bytes

d;f(*s,*r,n){for(*r=n>2?strcmp(s,++s):0;*s;*r*=!d|d<1^*r>0)d=strcmp(s-1,s++);}

Try it online!

-4 bytes thanks to @ceilingcat!

Returns:

• < 0 for Increasing Wavy
• > 0 for Decreasing Wavy
• 0 for not Wavy

C (clang), (削除) 82 (削除ここまで) 78 bytes

d;f(*s,*r,n){for(*r=n>3?strcmp(s,++s):0;*s;*r*=!d|d<1^*r>0)d=strcmp(s-1,s++);}

Try it online!

-4 bytes thanks to @ceilingcat!

Returns:

• < 0 for Increasing Wavy
• > 0 for Decreasing Wavy
• 0 for not Wavy

C (clang), 82 bytes

d;f(*s,*r,n){for(*r=n>3?strcmp(s,++s):0;*s;*r=d&&d>0^*r>0?0:*r)d=strcmp(s-1,s++);}

Try it online!

Returns:

• < 0 for Increasing Wavy
• > 0 for Decreasing Wavy
• 0 for not Wavy

C (clang), (削除) 82 (削除ここまで) 78 bytes

d;f(*s,*r,n){for(*r=n>2?strcmp(s,++s):0;*s;*r*=!d|d<1^*r>0)d=strcmp(s-1,s++);}

Try it online!

-4 bytes thanks to @ceilingcat!

Returns:

• < 0 for Increasing Wavy
• > 0 for Decreasing Wavy
• 0 for not Wavy

C (clang), 82 bytes

d;f(*s,*r,n){for(*r=n>3?strcmp(s,++s):0;*s;*r=d&&d>0^*r>0?0:*r)d=strcmp(s-1,s++);}

Try it online!

Returns:

• < 0 for Increasing Wavy
• > 0 for Decreasing Wavy
• 0 for not Wave

C (clang), 82 bytes

d;f(*s,*r,n){for(*r=n>3?strcmp(s,++s):0;*s;*r=d&&d>0^*r>0?0:*r)d=strcmp(s-1,s++);}

Try it online!

Returns:

• < 0 for Increasing Wavy
• > 0 for Decreasing Wavy
• 0 for not Wavy