Box 5 on SSA-1099 form includes Medicare B premiums. Do they count as income?

No, Medicare B premiums don’t count as income, per se. 

However, the amount in Box 5 of your Form SSA-1099 – including Medicare B premiums, if present – is used in the computation of the taxable Social Security amount included on your return.  

Please see the Social Security Benefits Worksheet—Lines 6a and 6b on page 31 of the 2021 Instructions for Form 1040 to see how taxable Social Security benefits are computed using Box 5.

Not exactly.  The income is the amount in box 5.  The Medicare is a deduction like tax withholding.  It will flow to Schedule A as a medical deduction if you itemize.