Is it bad that my porcelain tiles were stored outside in the rain?
If they are true porcelain, they have not absorbed water.  Porcelain are rated for exterior applications.

Some people generically call ceramic tiles porcelain, which may not be rated for continuous water exposure.  

The porcelains will completely clean up and not harbor anything other than a surface covering of mold, which might have come from the packaging.

Update

Tile Council  of North America (TCNA) Porcelain Certification


  Porcelain tile is defined as an impervious tile with a water absorption of 0.5% or less 
   as measured by the ASTM C373 test method. There are, however, many instances in which 
   tile not meeting this standard is advertised and sold as porcelain tile. 
   Marketplace confusion, installation problems and even liability concerns can result 
   when non-porcelain tile is misrepresented in the marketplace.