All perfect praise be to Allah The Lord of The Worlds and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
Straightening rows during prayer is permissible so long as the manner used to achieve that end renders the prayer valid. In addition, straightening rows is not a Bida'a since the Prophet (PBUH) used to do so.
Actually, Bida'a is adding a new devotional act; whereas, straightening the rows by drawing lines on mosque`s carpets doesn't include such an addition. This is because whatever is needed to fulfill an obligation properly is by itself an obligation, and the same ruling applies to acts of Sunnah. Besides, it is worth mentioning that Muslims nowadays have added many things to their mosques to facilitate performing prayers conveniently. For example, they have introduced carpets, which weren`t used at the time of the Prophet (PBUH), and the same goes for fans, air conditioners, water coolers…..Etcetera.
In conclusion, Muslims should not be too sensitive about anything that might help them perform acts of devotion properly. And Allah Knows Best.