Can I get Annual Solar Access percentage by averaging the percentages from the 12 months?

No.  The “monthly” solar access numbers are the solar access values for a given month. “Annual” is the solar access value for the year. Note that yearly would NOT be the average of the monthly percentage numbers, since each month gets weighted by its monthly insolation. For example 50% solar access in December would have much less impact on annual solar access than a 50% solar access in June (in the Northern Hemisphere). For any time interval, you need to add up (integrate for that time period) all the shaded insolation values, then divide by the unshaded insolation values integrated over the same time period.