I read that fosfomycin is not as effective as other antibiotics. Should I ask my doctor for another antibiotic?

Depends. If you are getting better, then no! People vary, and depending on if a culture was sent, might have lab data that demonstrates susceptibility. Don't worry unless not improving.
No. Fosfomycin is effective against certain types of bacteria. Every bacteria is sensitive to some antibiotics and resistant to others. Your doctor is choosing an antibiotic based on what bacteria you have.