Is it possible for too much masturbation cause thinning of hair?

No. It will not. It's excessive when it's taking away time from your relationships, work, study, sleep or whatever else you know you need to be doing. It's essentially universal among younger men. It will not make you bald, hairy, smooth, short, infertile, an unsuitable husband or father, insensitive, or unspiritual.
No. Masturbation in moderation is not harmful, unless it is done to the exclusion of normal daily activities. Furthermore, if you have a sexual partner excessive masturbation can make it harder for your partner to cause you to have an orgasm which may cause distress for your partner. Hair thinning is likely genetic, elevated Dht or hypothyroidism none of which are affected by masturbation.