Note that that doesn't really make your coins any safer. The seed phrase was already generated in a hot wallet and has been stored on a device with internet access for a period of time. Deleting the wallet app might also not actually delete the wallet file or other data, and it certainly won't overwrite those sectors of your phone's storage. If you want your coins in an offline wallet with the seed phrase only stored on paper, then you need to create the seed phrase and wallet using an airgapped device in the first place.

No wallet can guarantee 100% safety, and every wallet will only be as safe as the person using it. However, any open source software which is extensively and continuously examined by thousands of people, such as Bitcoin Core or Electrum, is likely going to be far more secure than some closed source wallet like Coinomi which makes basic errors such as sending your seed phrase to third parties and not encrypting its communications.