Is it a sin? No it is not.
BUT, I would question the depth of your faith, if being a Catholic Christian is embarrassing to you. If the rituals of your faith make you uncomfortable, and you do not want to openly display that you are a member of this church, you may want to explore why that is.
I came from a faith where Roman Catholics are considered to literally be spawn of the devil. They believe that the Pope is the Devil Incarnate on this earth. My beloved maternal grandfather was a minister in that church for over 55 years. I virtually adored that man, loved him with my whole heart.
I told him when i decided to become a Roman Catholic. I was nine years old, and I was afraid to tell him. I knew that he would be disappointed, and I was afraid that he would despise me and totally reject me. But, I told him anyway, because I loved my new faith, and I knew that it was more important than anything else in my life.
He did not reject me (I suspect he prayed for my immortal soul everyday till he died). He merely said that each person must make their own choice about their religion. He stopped my uncles from jumping all over me (they had). I was 13 when I was finally baptized, and he was dead. I suspect this would have upset him a bit, but I didn’t care. God was more important.
I have NEVER been ashamed to be a Catholic. I wore religious medals around my neck during my 10 years on active duty in the US Army. I had to take showers with groups of guys, and I ALWAYS had at least one medal, and a cross, around my neck. I was teased about this by some, asked questions about it by others, and most just ignored it.
I made the sign of the cross when saying grace before meals (admittedly very hurriedly during Basic Training, because we didn’t have much time to eat.). I even said grace before eating C-Rations out in the field. Sometimes guys would comment about it, most of the time nobody said anything.
I also said morning and night prayers. Even out in the field, I said my morning and night prayers. If necessary, I would say them while on guard duty, or doing some other military necessity, but they got said. I ALWAYS made the sign of the Cross when I said those prayers.
If anyone asked, or made a comment, I would explain that my faith expected me to give thanks to God for my meals. When someone would make a comment along the lines of “You call this garbage a meal?”, I would explain that it was nourishment for the body, and therefore something i should be grateful for. I explained that making the sign of the Cross was something that we did, to acknowledge the Trinity before and after prayers.
I know for a fact that a couple of guys sought out a Chaplain for some religious training because of my example.
I also got of those guys to help build a Chapel for a Convent near our Post in Bamberg, Germany. Sounds strange, a bunch of GI’s volunteering their labor (and money) to build a Chapel for some Nuns, but it actually happened. Check the European Stars & Stripes for May 1964, for details. The story was also in the local Bamberg newspapers in April and May 1964, the Overseas Weekly, the 4th Armored Division Newspaper and several others.
You NEVER know when your public example may impact someone else.
NEVER, EVER, BE ASHAMED TO BE A CATHOLIC CHRISTIAN.